Kinda what I hope the Op clarifies.
mountain biking and mountain bikes mean different things to different people. Some ride very technical trails and need a certain degree of bike for that while others ride little more than easy gravel.
No judgements, but if all the Op needs is a winter beater then the bike they already have will do.
I think in terms of platforms, and what their limits are. If you are easy trail/path riding then an older platform will meet the need for less cost. Even if one replaces components as they fail it would take years and years to match the cost of a modern bike, considering that one hopefully wouldn't buy the cheapest new bike available.
If ones aspirations are more aggressive in the technical sense then an older bike, as a platform, has limitations. You can upgrade parts but can't easily change the basic geometry or wheel sizing/mounts. To ride modern technical trails one probably wants a more modern platform to bolt parts onto.