[user5994461]

Their costs is already in the hundreds of million of dollars. They could hang on to all videos with no issue whatsoever. That's financially easily doable within their budget, and that's technically easily doable because google storage will take any amount of TB thrown at it.

The only fair assumption is that they store all videos. Remember that they are an ad business.

If it's too annoying, the wise decision would be to cut storage to only 1 month, or store only 1% of randomly selected videos. It will never be to stop storing videos.