You're not scanning through bulk storage like that all the time. Indexes have to be managed to be useful too which is another cost.

Videos still need to be indexed, why would Google keep the videos without indexing them?

Let’s not confuse the videos with the index of the videos. Videos can be stored on the slowest and cheapest medium, indexes might be kept in memory or flash. Each video takes hundreds of mb compressed, an entry in the index could be constant per video

Indexing and storage are two different things. If something if not accessed regularly, then you don't want to index it. You can still access it, it just takes longer to find.

Okay, so the videos may become preserved but inaccessible.

Google might have thrown out some old pages, but the internet keeps growing exponentially. I highly doubt that Google is indexing less pages than a couple years ago.

The Google indices are very expensive to create and serve. For one, they use a ton of memory.

YouTube has TPMs whose only focus is making storage very cheap.