Due to the high volume of writes flash is probably also not viable as flash has write limitations so I imagine they'll always stay on spinning disks
That would assume they actually delete things. Flash write limitation only comes into play when you write, delete, write, delete, write, delete, over and over again, hundreds of TB per drive. If all you're doing is archiving, that's one write cycle, and then it just sits there forever being occasionally read from.
The charges in high density triple layer flash cells dissipate in time frame measured by the low thousand hours. They need to be cycled around and that does impose a limit on their lifespan. As cold storage they are worse than tapes.
There is a lot of FUD in this post...
First, the "low thousand hour" charge dissipation time is for:
1) Unpowered drives, it doesn't apply when the drive has power
2) Only applies to drives that have already hit and exceeded their write cycle limit
3) Only applies to drives being stored at extreme temperatures
Yes, if you take an SSD, abuse the hell out of it for years and manage to hit the write limit, then remove it from the computer and throw it in a box, and then put that box on a shelf in a 50C warehouse, the data will likely decay and become unusable after a few weeks/months. That's a pretty extreme use case though.
I don't know what you mean by "cycling around" the drives reducing their lifespan...all you need to do is power on the drive to get around this decay clock, you don't need to write anything, you don't need to move anything. And who said anything about using flash media for "cold storage" anyway? I don't think anybody has suggested Google would archive PBs onto SSDs and then throw them in a box on a shelf in a warehouse for years, it wouldn't make any sense.