I think Inbox was an interesting approach to "fixing Gmail", which I've long had un-cite-able suspicions of internally being horribly broken (in the sense of maintainability and elegant architecture, not visible bugs).
Amongst only 11 outages in 14 years of continuous operation and use by probably a billion people, in 2011 Gmail lost 0.02% of users' email due to "a storage bug" that nuked all copies of data, and they had to restore from tape: https://gmail.googleblog.com/2011/02/gmail-back-soon-for-eve...
This status quo and example, along with the fact that https://google.com/appsstatus lists Gmail at the top, is a clear demonstration that Gmail is probably locked down so tight _everything_ has to be demonstrably proven to be at near-aviation-grade reliability before it's rolled out. I wonder if the Gmail team retains the same people from 5-10 years ago to minimize the amount of onboarding churn and maximize the chances things will be broken from unfamiliarity.
The Inbox idea was pretty inspired: _start again_, make a separate property/"brand", keep it reasonably niche, and you can get away with an effectively-lower SLA. And then once you get past the teething problems and speedbumps you can pivot the functionality back into Gmail.
But yeah, doing the pivot/fold-in does nuke the identity that got created. That makes power users who liked it more sad. And obviously you can't tell everyone the project is temporary or it won't go viral.
These systemic issues are not at all unique to Google, of course.