Has that happened with Mastodon?
Orgs spending volunteer money have to be careful, they have to allocate money to their stated causes or they could get in trouble. A Lemmy instance would have to coincide with their agenda.
A philanthropist can do what they want, but they could still attract criticism for not donating to world hunger or some more optics-friendly cause. They'd also probably end up with a fairly popular instance which would require effort spent on maintenance and moderation.
I think people who actually want to run instances will end up running them. I'm considering starting one. Some of those will end up running really good, stable and desirable instances which can then attract donations for the cause.