EDIT: I also used Service-Workers in the same project to provide offline functionality :) It worked great, I was suprised how good it worked!
> AIM, ICQ, IRC, and Yahoo Messenger all used local clients.
A webapp with a service worker is a local client. And a lot of people use Whatsapp Web, or Facebook Messenger's web site. I know I do.
If by "don't care" you mean "don't care enough to abandon a platform their friends are on" then I guess I agree. But all you've proven is that network effects matter.
People don't want something until it happens and they see it and say hey that's what I want, unless you are going off the assumption that people are sheeple and can be made to want messaging applications in the browser because they have been told to want it somehow.
Nobody did that, ever. It's simpler and cheaper to ship an in-browser solution, so everyone does that, that's all.
good luck trying to convince google of that. they are shaping the web just the way they want it. push notifications? nobody ever except the marketing apartment thought that was a good idea to add to anything. if we try making everything on the web like mobile apps we're gonna get garbage features like this and false expectations on what the web should be like.