It could continue running on HTTP(S). Did you know browsers and OSs can handle different URL schemas than the ones they natively open (http, https, file, data)? Ever saw a mailto, magnet, ms-word etc. URL schema? They can be opened with an in-browser or native app of your choice, and this has worked for years. Yes, clients would need to be patched for support but that’s easy. I would only add “instance’s native UI” as a fallback for people coming from outside the fediverse.