Big Sur should see at least another 18 months of support. Admittedly, that's just guessing based on past behavior, not a guarantee, which does kinda suck, but in fact it will probably see that much more support. Will likely remain safe to use for some amount of time past that, though who knows how long. Realistically, and accounting for 18 months being a bit pessimistic, one might reasonably expect another 24-30 months of use with a machine that can't upgrade past Big Sur, before having to make some kind of choice (install Linux, install Windows, or buy a new machine, I guess)