The R7000P model supports MU-MIMO. I was not aware that devices have to support it, too. This is something I will keep in mind. I thought MU-MIMO may be a good future-proof technology, but based on your response, this does not seem to be the case.
It would potentially be future-proof. Future proofing means it will work (or might work) well with or better with with future technology you might get. And that is the case with MU-MIMO. MU-MIMO is relatively new. As a result, not many devices support it...yet. But, that will likely change over time. My key point is that RIGHT NOW, it likely will not do you much good as your current devices that would connect to the router almost definitely do NOT support MU-MIMO.
It seems MU-MIMO technology is inconsistent as a result of a lack of an industry standard. Is this correct?
Nope. MU-MIMO is the result of an industry standard (i.e. the 802.11ac standard). And the 802.11ac standard is tighter than the 802.11n standard (at least as I understand it), so MU-MIMO implementations from different companies should more likely to play nice with each other. This might be due to you defining standard differently than how I am using it. For me, a standard is an approved document that outlines what is needed in order to achieve something (in this case the 802.11ac designation). Standards can be written loosely (i.e. allows for some wiggle room on how it in interpreted which might mean different people interpret it differently and thus implement it differently) or very tightly (i.e. does not really allow for wiggle room on how something is interpreted which means it is more likely to be implemented in the same manner).
To give an example from my world (in real life, I am a structural engineer who is also very good with computers), there is A36 (a structural steel standard) steel and A992 (a different standard) steel. A36 is a rather loose standard. It does specify things like acceptable minimum strength of steel and other things about the steel. It does not, however, specify a maximum strength of steel. A992, OTOH, does have specify a maximum strength of steel. Now, this might not seem like a big deal as one would think that stronger steel is always better and for your typical building in a non-seismic zone, it is not. But, when you get to seismic design, you need to know what your maximum strength of material is as seismic design is not just about strength, but also about something called ductility (basically the ability to absorb a lot of energy). The end point is that A992 is a tighter standard in many ways than A36, but in this particular case it is tighter when dealing with strength requirements. So, as a result, more steel samples might be able to satisfy A36 than A992.
Now, as I said, that may or may not help you understand what I mean by a loosely defined standard vs a tightly defined standard. It probably does not help at all with how loosely the standard might be would affect MIMO. For that, let me offer the example of language. Think of all the people who speak English. Now, some of them speak with an accent that is different than others. And some of those people might speak with a VERY HEAVY accent that is tough for many people who don't speak with that accent to understand. So, if you think of the English language as a "standard", then it would be a very loosely defined standard. And so if you have two people who are speaking with one having a VERY HEAVY accent that makes them hard to understand, some of what they are saying might be "lost" (so to speak). This would be kind of like that same thing with MIMO that is implemented slightly differently. Even thought both devices might be able to "speak" MIMO, if it is implemented differently enough, one might be "speaking with an accent" that is tough to understand, which then means that MIMO might not be working. Does that make sense?
Now, MU-MIMOs presence in device is "inconsistent" (i.e. not common) because it is a rather new technology that effectively just came on the scene in the last year or two. And like all new technologies, it takes a little while for it to become prevalent. It is more likely that you might end up seeing it in smaller devices like smartphones and tablets than SU-MIMO as I understand it is cheaper to implement than SU-MIMO and easier to fit into smaller devices than SU-MIMO (less hardware is required for it as I understand it).
Now that I understand more what MU-MIMO is, does it have practical applications in the real world? For example, would it make service interruptions less likely when two people are streaming ultra HD or even full HD content simultaneously?
Its practical application is that it would help with the situation you are dealing with. _IF_ all the devices connecting to a MU-MIMO router support MU-MIMO, then that router can truly serve up data simultaneously to multiple users means much less chance of interruptions, etc. But, again, this would require the devices that the users are watching videos (or doing other stuff) on all support MU-MIMO in addition to the router. This will NOT happen with most (if not all) currently owned devices as MU-MIMO is still relatively new and thus likely not supported on currently owned devices. But, more and more devices likely will support it in the future.