Assuming Jim Henson is the legal creator, it will enter the public domain in 2085.
If the copies survive
All 7 seasons are well seeded on the torrents. It's hard for media to become lost these days, but if you want to see a lot of it, you aren't going to do so legally under such a ludicrous IP regime.
But well before everyone had enough data storage in their pocket to hold all those episodes and much more.
Also, before recording from tv (or streaming) was deliberately made difficult by nonsensical DRM...
Out of curiosity, is there anything currently on TV/streaming that you can't find a de-DRM'd copy of online?
Ok, but are they of the same quality as masters?
Definitely not. They appear to be SD rips of television broadcasts. Obviously it would be best if rights holders made their media available for reasonable prices on reasonable services, but until we have sane IP laws, hobbyist preservation efforts will be the predominant way that some media will be preserved for consumption at all, quality notwithstanding.
It's hard for popular media to become lost these days.
Undiscovered or not-mainstream-popular work gets lost all the time.