I think they will/are using it for AI/ML training. After all, it is data created by a human, so useful in the newly polluted sea of AI generated content.
Doubt data uploaded 10+ years ago has the necessary agreements in place, at least in countries like Europe/UK requiring user consent for processing of private data.
Collecting data for one purpose (video sharing site), but then using it for another (training AI) is very much verboten.
Keeping all that private and never watched video ready-to-serve must cost so much, with zero revenue.