I do have a few contentions with your assessments.
I got my Windows Phone in the summer of 2013. It was WP8.
It had pretty fast notifications. In fact, I used to get real-time CNN notifications and I got information much faster than when what show up on the news channel. I even got wind of Nelson Mandela's death before Wikipedia made any edits and any obituaries made it to Google's first page.
The browser was Internet Explorer 11. It wasn't the best, but it performed well enough for reading articles.
Apps. I agree and that's still a problem almost a decade later.
IM was actually one of the strengths of the phone with the People Hub/Rooms feature. You could use Facebook, Twitter, and LinkedIn all in one application. I don't know why they removed in Windows 10 Mobile.
The games were mostly iOS ports, so quality varied, but many were well-known. Angry Birds and its myriad variations, Fruit Slash, Candy Crush, etc. It even got GTA San Andreas. But a distinctive feature for achievement junkies was that many of those games had Xbox Live integration (still works too). The phone also worked as a touch controller and remote with the Xbox 360 and the Xbox One.
You're right that it wasn't the most capable phone. But it offered a lot more than a refinement of the basic essence of a phone.