IE11 support will go away at some point. Many of those apps are now being updated to work on Chrome -- I know of a few large shops that aren't testing IE or Edge anymore for internal apps!
I do agree that they did a heroic effort on making the update process incredibly reliable. Most non-prehistoric applications transition seamlessly.
Note that I personally equate Windows 10 and the subscription "Office 365 ProPlus". The layers of complexity are astounding. I present to you what I call the "Office 365 ProPlus Build Decoder Ring" - https://support.office.com/en-us/article/Version-and-build-n...
Windows 10 itself has about 8-10 releases that an enterprise needs to think about. Transition from release to release has gotchas and additional Windows-Office gotchas. (For example, the way Office authenticates changes in certain builds) (See: https://www.microsoft.com/en-us/itpro/windows-10/release-inf...)
I haven't seen any issues with apps though, so I have to dispute this claim. We just moved 5,000 employees from Windows 7 to 10, and it went far smoother than anyone really expected - everything just works. It's a wide range of apps too, everything from in-house WPF apps to CAD and engineering software.