Backwards-compatibility is usually only 1 generation, so it just pushes the bitrot forward 5-10 years.
Relying on original manufacturers to re-release old games depends fully on that manufacturer's ability and willingness, and up to now the only manufacturer doing it, Nintendo, has been very selective in which games they re-release. And it's a separate release per console so again only pushes the rot for that game forward to that console's lifespan.
Thanks to emulators I'm willing to bet that in 100 years you will be able to play currently-emulatable games since I expect a good Windows/PC emulator to exist at that time.
Why would someone in a 100 years be using an emulator from today (say SNES) inside a Windows emulator? Seems like if there was any interest at all some folks would write a native SNES emulator in the native hard/OS of the time. There's nothing about Windows that makes is especially ideal for emulating consoles for the last 40 years.
I think every console manufacturer should release their own legal emulators; companies like Nintendo could make a lot of money off of a NES or GB(A) emulator on smartphones, they can keep selling their classics for $1-$5 apiece.