They already do though, kind of. Their "Community Points" are an opt-in crypto shitcoin that the mods get a chunk of every month, and they're free to sell them to anyone willing to buy.
And there's always someone willing to buy, for some reason.
Just look at /r/cryptocurrency and their MOON token. Mods get something like $2000+ USD worth per month. I'm always surprised that 99% of reddit has never heard of them.
They can do what communities like /r/cryptocurrency and /r/fortniteBR do and opt-in to Community Points. Mods of the former make a decent killing with their MOON token.
Granted it's done by milking their users for dollars... but if it's good enough for reddit the company!