That said, surveying today's landscape, it would appear that there isn't actually a huge amount of demand for either full resolution user-curated sites focused on photography or RSS readers. The vast bulk of users are fine with throwing their baby and cat pictures up on more casual sites like Facebook and Instagram or getting their news from Facebook/Reddit/etc.
Problem wasn’t full resolution, it was re-compressing your originals. Photographers that care about full res, care about original quality.
While there were (and still are) options like SmugMug for photogs, Flickr was the only prosumer one supporting unlimited original quality.
Also on the “stagnating” comment — at some point is it possible you’ve hit a reasonable photo site, and shouldn’t tinker much?
Turns out photographers who posted and curated 50,000 photos are not big fans of having the rug pulled out from under them by people in marketing trying to change the UI for their own engagement metrics.
I actually think that's somewhat fair. I could come up with some things that I wish were better with Flickr and there are probably lots of things I'm not thinking of. But I still use it and, in conjunction with Lightroom, I find it's a pretty good way to display a curated collection of edited photographs (as well as serving as a backup of last resort).
I did object to the big UI change a few years ago and still don't really love it. On the other hand, I don't actually use the web interface much. I upload from Lightroom and I mostly view from a tablet.
The machine learning assisted search. For example, if I want to find photos of whiteboards that I took to record a meeting, I type in "whiteboard".
Making it even more ridiculous that they killed off Google Reader for it (or didn't bother to revive it afterwards)
EDIT: Also, they already had Picasa Web Albums since 2006. I don't know what exactly Google Photos has to offer over it in terms of functionality.