Is that really so though? Assuming some small percentage of the user base chooses to donate, would that not scale as the instance gets larger, as long as the percentage stays about the same?

I would guess that it wouldn't scale in a linear fashion - earlier adopters who saw first-hand all the problems and what the admins had to do would tend to be more appreciative of their efforts than folks who arrive further down the line when everything is humming along.

That said, donating isn't the only string to the funding bow - affiliate links would tend to scale linearly with user numbers and I imagine we'll see bots or even hard coded regular expressions that spot links people add and tag an affiliate link on the end. If they are going to be used, instance admins might as well claw some cash back.