Yes, an app is to be treated as a disposable thing with a shelf life of a few years at best. But on the other hand, so are many "advanced" web apps that rely on vendor-specific implementations of specs that may or may not survive (not to mention HTTPS, certificates and their significance to browser behavior…).

If you want something to live, be prepared to keep it up to date with the platform it's running on - regardless of what platform that is. The most stable platform to target is likely Win32, specially with Wine.

I'd expect a simple HTML static document to continue working mostly as intended for a long time, but anything interactive and/or advanced is likely to bit rot IMHO.