I mean if the idea is to get people off of the matrix.org homeserver and onto smaller federated servers, this might accomplish at least half of that.

Yes, that's insane. I totally understand requiring payments for big attachments (even then I would probably set a bigger limit for individual attachments and instead charge for attachment/message retention a-la Slack)

But paid public rooms just means that people will jump ship from matrix.org; combined with lack of "portable identity" or any other way to easily transfer your account to another server, this may just be the beginning of the end for matrix, which is really sad.