Ideas like this have come up a few times. Unfortunately I can’t find the one where this point was made (maybe it was on Discord): plugin developers tend to be constrained more by time than money, and plugin income is unlikely to be large enough to pull them away from a day job (or college).
Obsidian already always developers to include donation links in plugin listings, and plugins are allowed to charge for functionality (tho payment is handled by the developer, not the plugin gallery); Remotely Save does this.
Here are some prior discussions: