I am clueless about the adoption of the plugin, but I hope you’re right and that there would be enough interest to make it worth @clavaque’s time to develop such paid add ins and maintain the main plugin.
I think it’s very important that everything resides in our database and payment processors only, so we actually own our audience, and the fact we are not stuck having to pay a (high) monthly fee (something affordable for upgraded users like 10 dollars a month would be ok IMHO, not sure what others think) to keep things running (I am against charging a percentage of our revenue like some other plugins or holding our user base hostile on locked in systems, though, paypal/patreon and other platforms already do enough of it and it’s almost impossible to retain all subscribers whenever you want a migration, you’re stuck forever).
That said, he needs to be able to fund his work too, hence why the paid add ons. To be sure there would be enough interest @clavaque should try to pre-sell them and only develop those pieces if enough of us buy those, otherwise he would be stuck spending time on things that aren’t appealing enough to justify enough users spending on those things. I might buy it to support the plugin in general, you might do the same, sadly most people don’t think this way, but depending on the size of any audience there’s always a percentage of good people who will even buy add ons they don’t necessarily need just because that’s what’s available (or he can even have a more evident donate button across the forum as well, maybe a tiny donate button on the top navigation bar so people always see it and are taken to a page where @clavaque explains why he would appreciate donations, even if small or sporadic, so his efforts are somehow rewarded).
As mentioned on another topic I made a donation a while ago and I hope more users do the same, even if they can’t afford the full price of upgrading to the pro version (I did that too, a long time ago).
Free software is great, people try it, they might use the basic version until they have enough return from it, then many will upgrade because of the value it brings.
I’d surely buy a $ 20 add on that could manage late payments better, for example (instant downgrades on payment declines and reupgrades with notifications for both admin and user on successful late payments).
Perhaps the new PayPal integration could also be sold as an add on, so you only get the previous framework available if you don’t purchase it or you pay 20 dollars to be able to use PayPal’s new API or you’re stuck with basic buttons. Also, it’s important to note that for the ones amongst you that don’t need to calculate tax depending on the customers’ location, you can just create a regular PayPal subscription button using their “non code dummy option”, then place those buttons on your site. When a user subscribes using those you can create/upgrade your user manually using Wordpress’ User Admin and the IPN works normally after that (I don’t trust the IPN to demote anyways, I usually set my EOTs manually each time I receive a payment decline notification, and s2Member won’t also re-upgrade when a demoted user makes a late payment either, so I rely on users messaging me to let me know they paid as my EOT email makes them aware of it).
Just some thoughts, of course.