I get a little concerned about this too.
I figure as long as they depart the branch, on a "good, solid build", then things won't be all that bad, but over time it's a security risk, if they don't patch the last build, which requires a code-branch for it, and a whole maintenance/test team.
We've got an HTC One M8 and a 1020 in the family, both of which should be moving forward, by spec, but may get "stuck" on .456 or whatever build is last released for the "big change".
It's a little frustrating (clearly, due to all the previous protest/discussion) to have two phones that out-spec current models, for the most part (M8 in particular), but are getting tossed aside because of "feedback", or that's the excuse, for not wanting to support more phones, IMHO.
I think my daughter has had 1-2 issues on the 1020 in 5 months, and my wife has had maybe 4-5, on the M8. Yeah, they function horribly on W10m builds, yep ;-]