Hopefully they don't go out of their way to try to lock you into using their VTT. I'm not a fan of Roll20 myself but as long as they let Demiplane be themselves and keep it mostly open for third party plugins to do their thing, we should be alright in the long run.
I wouldn't be surprised if they introduce another subscription tier at some point that features Ads or worse. Then, at a later date, they will gradually raise prices across the board so the new subscription tier becomes as expensive as the current one.