I view almost al things mechanically. Before I get into that, I have to think about something I heard from a friend years ago. He said that you can find it for free online if you look long enough.

As a mechanic, the only way I see Quora doing this conversion is if they completely restructure. They would have to adopt a similar model to Medium where users only get a certain amount of Q&A per month. Then they could impose the $5.00 fee. If they were to use a subscribe to "Spaces" as you suggest, then they would have pay out from ad revenue generated by views/members which is similar to FB groups.

That's the best I can come up with. Either way, there's no sense in arguing over speculation. We'll wait and see where things lead.