Ok, I see your premise now. I am not sure I agree: do you know of a system that has seen less investment on backwards compatibility yet successfully maintains it?
Of course, not "frozen" like TeX, but seeing active feature development too.
Pretty sure HaikuOS runs software originally compiled for BeOS in 1995. They have a very small development team and they are quite active today.
...for that matter WINE is even able to make Linux compatible with Windows applications to a great extent, and they certainly aren't backed with Microsoft's wealth of resources.
Of course, not "frozen" like TeX, but seeing active feature development too.