This isn't a question that applies across the board, as there's many needs, some requiring one or the other.
However, in the consumer, as well as high end business markets, NVMe SSD's are making huge leaps in a shorter span than the 2.5" SSD's has done. The turning point began for SSD usage due to the HDD's own false shortage, one that never was. To put it short, when there's a real shortage, yes pricing rises, do does wait times. The only thing I seen were HDD's at 3x the cost, with just as fast shipping times.
The SSD industry cashed in on this by the creation of a price war of their own, for most that purchased their first SSD for an OS drive, few has looked back. Many, to include myself, still uses HDD's for data and backups, and some has went 100% SSD due to later price drops for 500GB to 2TB models. And why not? With their high endurance rates, my first 128GB Crucial m4 purchased in 2012 still shows 98% lifespan. A highly underrated 180GB Intel 330 purchased a few months later that still reads & writes at over 500MB/sec at 99% lifespan, and several others purchased since then & no troubles. Why should I change to HDD's that may die in under six months (had to RMA a WD Caviar Black three months after purchase, and a WD RE4 5-6 months afterwards)?
I've yet to have the first SSD die on me, and as far as Flash storage goes, have had only one defective USB stick, it works, but only at a crawl & was a no name brand purchased on eBay for $5 years ago. If I had the same luck with HDD's as SSD's, may be telling a different story.
The NVMe SSD in this PC, a 512GB Samsung 950 PRO, was actually designed for high end Workstation use (or prosumer) has been totally outstanding in every way. Have a couple more similar (240GB MyDigitalSSD for a third of the price at $114), one still on original package in my safe, the other is nearly as fast as the 850 PRO, for the money am not complaining, although did have to address heat. This was fixed easily by getting a Sintech PCIe adapter with a small fan from Amazon, temps nosedived from 72C (peak) to 51C under same load. Therefore, my first choice will be the M.2 NVMe SSD where supported, otherwise the best 2.5" SSD's I can find on promo, also recently purchased a 256Gb Samsung 850 Pro for $99, place in the safe along with the NVMe model, to have one of each ready for duty.
That stated, also have a lot of HDD's of the Workstation/Server type in use & some also ready for duty, was stocking up on WD RE4's, now have went to their eventual replacement in the 2TB WD Gold, optimized for Workstations. Point being, if these can handle up to 550TB/yearly in a server, should handle my needs as a Home user just as well for only a few more dollars on promo. To hold down the number of start/stop cycles on my Golds & some Seagate models, which can rack up rather fast, used a trick that's been in usage for some time in the HIPM-DIPM utility. Which basically is a registry entry added then make the changes in Power Options > Change Plan Settings > Change Advanced Power Settings. Select Active, then at 'Turn Hard Disk off After' enter '0' (zero) & then OK. Reopen, it'll say Never. This means that for the most part, all of the starting & stopping will pretty much be in line with actual powering on the computer & shutting down cycles. Download links are below, should get both enable & disable while there in case one changes their mind, although many doesn't. It has to be merged again after a power change, such as Recommended to High Performance or any other, so be sure to save in a safe place.
Actually, this was meant to be more of a battery saver than to put an end to all of the start/stop cycles, few of these Tutorials suggests this for the purpose. Yet the wdidle3 utility only works on certain (usually older) WD HDD's only, was originally released by them for their drives & has been ran on all of my WD RE4's, the first of which was a RE4-GP (Green Power), the utility (small bootable ISO) made it into a regular 7200 rpm RE4. Once the command (wdidle3 /D) is ran & system shutdown by power button (not rebooted), this stops the parking every 7-8 seconds if no activity. Is likely commonly used, not too long back purchased an Optiplex 780 MT, the installed HDD, a SATA-2 model, has over 8 years of usage, yet just over 200 start/stop cycles when I checked, and most likely, some of those were of my own during setup. That's amazing, over 8 years of usage running wide open much of the time & still reads & writes at near 90MB/sec.
Kind of like brake pads, HDD's wears more during heavy start/stop cycles. Spinning platters doesn't wear much if a quality brand & assembled with quality all the way down the chain. It's the other parts that does, and why drives are rated to last a certain number of start/stop cycles, kind of like SSD's are rated to withstand a certain number of TBW written. Generally speaking, the lower the cost of both, the less advertised lifespan with both types of drives, be it start/stop (or load/unload) cycles for HDD's or TBW for SSD's.
Therefore, this is one reason why one size doesn't fit all. Many can go the life of the computer if a quality drive is preinstalled & apply the above trick to save wear & tear. On the other hand, others (like myself) wants the best of both worlds, eventually will convert this system (or my next build) into a NVMe/2.5" SSD system only. Or NVMe/M.2 SATA-3, although I prefer the 2.5" drives, I don't use either the Ultra M.2 or M.2 SATA-3 sockets on my motherboards, that's where the most heat is. Rather, since am a single GPU user, use the second x16 3.0 slot for the NVMe SSD, there's little impact on the GPU's performance. Especially not being a gamer, for that matter, some that are says very little loss. Not enough to justify installing their expensive NVMe SSD on a toasty MB for a couple FPS.
While I don't know when, if ever, SSD's will totally replace HDD's, the latter is rapidly becoming more of a storage medium. Even some middle of the road notebooks are now shipping with a SSD, and in well heeled models, a 512GB to 1TB NVMe SSD & a large SSD for storage. So the industry's changing.
Add to that, back when Windows 7 was released and a few years afterwards, upgrading the RAM was the most 'bang for the buck'. Not so today, if an SSD is not installed, that's the recommendation, since most systems of today ships with a minimum of 8GB RAM installed.
FInal thoughts, I don't feel that in my lifetime, will see HDD's become totally replaced, as these are excellent for data, virtual machine creation & for external backup usage. Price wise, it's hard to justify purchasing a 2TB SSD to become an external backup device, when one can have 6 to 8TB of HDD space for the same price across several drives, it's best to have several 1 to 2TB drives versus one large for insurance. One should never rely on a single drive for all of one's backups & data, if it goes, so does everything with it & recovery is an expensive, non-guaranteed process. That's why I rotate externals with every backup & for items of importance, scatter on more than one HDD, also make use of USB Flash drives, optical media & cloud storage, the latter only for photos & some non-critical items such as frequently used drivers, service packs & certain Windows Updates needed for a clean or reinstall of Windows. I have a smaller, slow (SATA-1), yet still good running HDD to store my Windows & Linux ISO's for when needed, it's also imaged from time to time.
HDD's are still here to stay for a long time to come.