Copy & Paste from MDN:
There are some key principles a web app should try to observe to be identified as a PWA. It should be:
Discoverable, so the contents can be found through search engines.
Installable, so it can be available on the device's home screen or app launcher.
Linkable, so you can share it by simply sending a URL.
Network independent, so it works offline or with a poor network connection.
Progressive, so it's still usable on a basic level on older browsers, but fully-functional on the latest ones.
Re-engageable, so it's able to send notifications whenever there's new content available.
Responsive, so it's usable on any device with a screen and a browser—mobile phones, tablets, laptops, TVs, refrigerators, etc.
Safe, so the connections between the user, the app, and your server are secured against any third parties trying to get access to sensitive data.
Offering these features and making use of all the advantages offered by web applications can create a compelling, highly flexible offering for your users.
Each instance could have it's own PWA. This may seem a little fragmented but, I doubt many regular folk will notice at first. The site they're on will seem more professional, complete, and modern which gives GS the edge and makes it more competitive so people have more reason to #deletefacebook. The application icon will sit silently luring them to participate in the fediverse.
Maybe it'd be possible to have on PWA that has an easy way to log in to any given site Os/AP site? And be launched multiple times for people with multiple accounts. Unsure if that's better or worse. &S, Fedilab, Choqok or other proper desktop/mobile applications are possibly better suited for this despite not sharing the instances theme.
However, desktop applications need development help, package maintainers, and I imagine some UX/UI help. (i.e. A "new" version of Choqok was released several months ago and nobody has updated the Ubuntu PPA and I can't get it to build properly from source. The old one functions and is Qt5.)
A PWA would reduce the need for these other applications and each instance could have it's own official app that the user's browser prompts them to install.