Those 40 hours of black screen probably compress pretty well. If that's not good enough, maybe reducing old/unwatched content to 480p would be a good alternative to just purging it?
Also, 4.3PB per day, at today's newegg HDD prices, is about $100k per day or $36.5 million per year. Yeah, you have to consider replication and whatnot, but you get the point-- this is pocket change for Google.
Acquiring the SSD isn't the only cost, it's also paying for power, the cost of the buildings they are housed in, the staff to look after such hardware, hiring SREs to maintain availability etc which will also make it higher.
In all likelihood, too few people would be willing to pay for this long-term to be worth implementing it. And it would increase the chances of an alternative platform becoming successful, more than just deleting old videos, because having to pay for something that was free before tends to cause a bigger outcry, and also because it would give people more time to migrate their videos.
Blame all the fuckwits of the world for uploading yet another "40 hours of black screen with a metal pipe sound randomly through the video" video.
I saw they take in like 4.3PB a day but that post was from years ago so it's probably even more, now.