Donation-based models have been used for ages and it worked for mastodon.world
Maybe I wasn’t clear on the blog post. There are two objections to donation-based funding:
unless everyone working on an instance is properly compensated, it’s hard to say “it is working” or "it is sustainable"
it may work for particular instances, but it stunts the growth of the overall fediverse.
You might not see it that way, but my argument is that relying on donations hides the true costs of running the server from the users and (like in ad-funded business) distorts the “market” in a way that makes the overall system less efficient.