I think it is very very sad situation that Sketchfab must be in. I understand that when they change their politics and plans any new customer must agree to that, and excisting customers must accept that if they don´t upgrade any further models will not have the funcionallity that demand an upgrade. But that they tell people that have supported them since the start will loss the funcionallity on models that have been uploaded on a paid plan years ago, I think that is very bad handling of excisting customers, they must be in in a very bad situation to do this.
I would not go for any plan further on with Sketchfab