I think the one reason that got developers so hung up on the application server model is because everyone assumed that the client needed to have an inbox URL to listen to requests all the time, which led to the idea that AP *requires* the push-based flow.
This will hopefully change this year, though. I started working on the ideas I've laid out on https://raphael.lullis.net/a-plan-for-social-media-less-fedi-more-webby/ , I just hope I don't go broke before I get something usable out of it.