I feel like that's a risk for all software, OSS or not. If the OSS maintainers stop updating your software, it probably stops running on the latest version of your OS at some point. Apple or Microsoft may accelerate that process (Apple almost certainly, Microsoft less so as they're famously very concerned about backwards compatibility).

Obviously there's some software that probably could keep running without updates almost forever, but even for simple things if you're dynamically linked against 32 bit libraries is going to have a hard time on 64 bit OSes if the 32 bit versions of that library aren't available. OSes may add or remove functionality your app relied upon. Heck as long as we're imagining a world where Adobe has decided to stop making photoshop, we could also imagine a world where IPv4 is finally deprecated and removed. How many applications out there assume an IP address is and will always be 4 octets?