It is possible.
Could a top member of chess.com defeat an old GM of the past, 60's or 70's?

Statistically speaking, I "could" beat an old master; it is just very unlikely. So, yes, it is possible.
Actually, there are some heavy-weights on chess.com: Dzindzi, Shanky, Khachiyan, Lenderman, Kayden Troff, etc.

I need help convincing Carlsen to join chess.com. Someone, please persuade him? That would settle the question permanently. Hmmm, now I wonder if there is a reason Carlsen is not on chess.com...

But the idea that someone 2000+ here (me, for instance) could beat Korchnoi! His old French Defense games are still cited in modern books.

Old guys can still play! I played a 93 year old expert last week who had played Tartakower and Koltanowski and although I won he played some strong moves. It depends on the person, the game, and the style of player. The oldest player who I've ever met had played Ed Lasker.
I thought a player who reliess less on calculation may not lose much strength at older age. Notably Lasker and Smyslov were still world class players in their 60s.
After reading the article "How to Play Against Old Guys, I wondered about if any top player in chess.com (+2000) could defeat Korchnoi or another famous old GM of the past (60's or 70's), cause old players get tired, lose concentration, miss basic tactical shots due to tiredness or can fall into a state of passivity...