It seems you understand SA perfectly well
You're right, SA doesn't prevent paywalling. But that's just a technicality in my view because only non-NC non-SA CC licensed content can be relicensed to something that doesn't permit free distribution and then sold. This is the primary reason I think basically no one is going to even attempt to sell someone's SA content any more than NC content. Of course, I'm willing to have my mind changed on this if there's evidence to the contrary...
I'm not sure if there would be any benefit even in the short term. Who would pay for a Patreon that only offers freely distributable content made by other creators, especially when that content is already available for free on the Hub? You'd have to have an incentive to join someone's Patreon, and if all they're doing is saying "Here's a release XYZ (CC BY-SA) by atani, join now!"... is anyone going to join? The creator has to clearly mention the license of the content they're selling because attribution is part of CC license terms, and mentioning the license is part of the attribution. If they don't provide proper attribution, you can nail them on that just like you'd nail them on selling NC content.
The only way I see SA licensed content being actually vulnerable to being paywalled is the same way as with NC content: by breaking license terms. In the case of NC, it's the paywalling itself that breaks license terms, while in the case of SA content, it would be relicensing (to PC or any other).