The majority of miner revenue is a subsidy given via inflation. It's effectively paid for by all bitcoin holders, and is decoupled from usage and fees. Fewer people transacting is going to make no difference. Miners aren't going to suddenly think "Oh, perhaps I should mine less now that these clowns at Wikimedia aren't taking donations."

This is the case now, but every halving shifts that proportion towards fees.

Correct, but miners are operating on the assumption of revenue now, and not 8 or 12 years down the line when transaction fees eventually exceed the block subsidy.

By that time, the network effect will be so large that Wikimedia will not be able to just ignore it to pacify a small group of whining overgrown children.