If you look at the bug fixes to any Windows update there are a lot of them.
I am wondering if a number of machines will now be taken from 1607 to 1709 without ever having had 1703. It's certainly possible, but nothing official has been said.
Microsoft has been struggling to get to their target of 2 feature updates per year on Windows 10 since it was released. I will be the first to say that Versions in the 15XX range were definitely late beta, regardless of "the official position." Edge still remains horribly immature for a web browser and, at least on my machine, is generally laggy, badly laggy.
I don't think that anyone other than those that were the decision makers regarding Windows as a Service delivery model can tell you what the discussions and justifications were. I think it was a combination of things, several of the most important being ability to add features much more quickly (which users [some, never all] are demanding), to be able to enforce updating of the OS and, thus, having a far more consistent configuration "out there in the wild" than was ever possible when the end user was allowed to pick and choose among updates, and to respond to certain security issues more rapidly, and comprehensively, than traditional patching allowed.
To describe the roll out of 1703 as glacial is an understatement. Of course, I suspect that a large part of this is (and will remain) based on what Microsoft is getting back via system health telemetry as version update cohorts are formed and the update pushed out. There is no way that Microsoft could ever have an in-house testbed that includes every possible system configuration on which people actually have Windows 10 running (and many of those systems were never "certified" for Windows 10 by their respective makers, either). There are bound to be unexpected issues that were impossible to predict based upon even the most thorough of in-house testing prior to roll out. If the issues are relatively easy to fix and/or don't cripple systems then the full roll out can continue relatively quickly. If, however, they aren't then that puts the brakes on the next update cohort until the fixes have been applied to the cohort having issues to see if they actually fix those issues on those machines. This whole cycle, even within a single update cohort where things don't go as expected, is a "lather, rinse, repeat" type cycle until the all clear comes in, then it's on to the next cohort, where other hardware and other issues might be encountered.
To be perfectly honest I have no idea how Microsoft (or anyone else, for that matter) could ever be able to write an OS that will "instantly run without issue" on the vast numbers of systems of vastly different configurations, but somehow, they do. I would not want to be on the Windows 10 development team for love nor money!
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story