Maybe this is already available or has already been discussed. If so forgive me.
What if Webflow had an active/inactive setting or archive vs. active setting? So if you had 10 sites, you could archive the 5 that you have already "finished" exporting and pushing out to another place. Then you could still keep a copy of them in Webflow, just not actively using them and they wouldn't count against your account of active projects. That way if you ever needed to you could activate the site, play with it in Webflow and re-export or something.
I think that would give people a chance to keep a certain number of active projects without feeling like they have to keep paying more and more unless they really do need to scale, which typically does cost more money. If you are staying at the same scale, but adding work over time, you could stay at a similar price structure, while archiving old projects.
Just a thought.