Honestly, for a bookmark manager, there is nothing innovative or advanced about having a nested structure that would merit being held hostage under the "Professional" subscription. This is a basic feature supported by virtually every modern web browser. A lot of folks who'd be interested in having a bookmark manager are likely already using a nested structure for organizing their own stuff. Charging for this actually deters me from using Raindrop at all.
If Raindrop wants to charge in some way for nesting, then maybe develop a feature that is actually worth paying for. Perhaps Raindrop could assist users in automatically developing a nested folder structure with groups based on the type of bookmark content (e.g., media, software development, etc.). The ability to make one's own nested structures is free to do in any browser. It takes time to do whether one is using your service or not. I'd rather not be charged for it--this should be a standard feature.