Google Inbox was great. Intuitive, modern, snappy, a testbed for new features, and on the surface it seemed less monetized and ad-ridden than gmail.
I switched away from Google's email-clients as soon as Inbox died, also having used Gmail since as soon as I got an invite when it was more closed than today.
It's fair because MANY people had never heard of it. I even heard on those same Leo Laporte podcasts people going "what's inbox", people who report on tech for a living. I imagine, compared to people just using the gmail app or the web interface, it had terrible adoption rates which made it not worth supporting. From all I could gather is it was almost exclusively used by those in tech and then only by a small subset.
I imagine reader had a small user base. RSS feeds are a pretty obscure thing and always were.
The only hangouts messages I've ever received have been group spam and likely has a relatively small user base.
Google Circles was largely spam and garbage content created for SEO in my experience and probably had a very small real user base.
Etc etc. If you your user base is a small percentage, or a fraction of your percentage, of your overall user base it's perfectly fair to go "well, let's shelf that".
Couldn't figure out how it was better than the stuff I already used and switched back to the regular Gmail client again.
To be fair, Gmail has been my primary email since early 2009 and I'd never even heard of Inbox until people on Leo Laporte's podcast started whining it was going away.