Storage for YT is not like storage for your computer. The question to ask is not if Google has enough hard drive space to keep old videos, but what it costs Google to keep all YT videos available.

First keep in mind that there isn’t just one giant server at YouTube. Everything is replicated onto many parallel servers. And enormous datasets that are too large for any one server are “sharded” across many. Perhaps it takes 1000 server clusters to store one copy of everything.

Now you have to parallelize copies of those 1000 so there are redundant servers that can scale up to meet viewer capacity. This is a server “grid.”

But only some videos are being watched millions of times today. Only those server nodes need 100x redundancy for scale. The long tail of less watched videos might barely need a single node to be kept available.

So there is a massive “head” of videos that need tremendous server capacity to be available enough, and a very long and thin “tail” of videos that don’t require much resources at all.

The “head” grows as YT’s overall audience gets bigger. It’s very resource hungry and is probably their main challenge.

The “tail” gets longer as the total library of videos grows. But the tail is thin and making it longer isn’t that expensive.

I’m sure there is also a threshold below which they will drop videos. Made over 1 year ago. More than 30 seconds long. Has never been viewed once. Auhthor account hasn’t been visited in a year either. Drop it. No one will ever know.