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".
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.