Right, the incentive will be to create accounts on the popular instances, not on the paid ones.

There was a spam wave some months ago. The spammers were using Mastodon and Sharkey instances that had open registrations. They were not signing up to paid-only instances to become "legit".

I meant spam as shorthand for "bulk send of messages of similar content". Cheap to produce and to send. If bots paid by some government count as spam, then we should also say the same about sponsored content or PR releases.

Anyway, my point is that charging for access to a network where access is already open is good enough for a filter to avoid spam originating from your node. Spammers are not going to be interested in paying $29/year to be able to send posts via Mastodon when they can just create a bunch of accounts on servers with open registrations or simply running their own botnet.