And when you consider that hard drives have stagnated in price over the past few years, YouTube is doomed if it does not delete unprofitable videos. Unless prices to host content goes down, the whole thing becomes a sort of ponzi.
The suggested idea, which Google might already do, is to keep the low quality videos in less redundant storage on the theory that they can be re-encoded from the more durably stored original video if the underlying storage fails.
Not necessarily, is storage(and not retrieval) that much of a cost? and if so is youtube's revenue/storage cost actually decreasing? and if it is would the backlash for deleting people's videos worth the reputational cost? especially when you have an entire industry around small people trying to get big, the majority of which will never get there.
Youtube even tried putting a limitation on non-premium users watching 4k video and pulled back on it, can't imagine something as drastic as deleting videos would be worth it.