I'm a lot more comfortable buying used SATA SSDs than used hard drives. The mechanical fragility of hard drives makes it hard for me to trust buying old ones. They wear out, or they lose their seal/filtration, or they seize up, or they get ruined in shipping.
I don't have that fear with SSDs. It's hard for them to get damaged in shipping, and if they've sat around for 10 years, that probably won't affect them much either. Some of the older SSDs even use a type of Flash chip packaging with pins sticking out the side, so they can be soldered (though probably not easily). So they're at least a bit repairable.
However, I'm more comfortable with MLC (by which I mean 2-bit) drives than I am with the current TLC (3-bit) standard. I'm not at the point of stockpiling MLC drives, but I prefer them.
Unfortunately, I wouldn't really be happy with SSDs in older machines, especially 1990s systems. But I think they'll be a readily available option for as long as any of us will be around to care.
Recently I've been thinking about a mid-90s build and have considered buying a 5400-7200rpm SCSI drive on eBay, but I can't shake the feeling the drives will end up being faulty. I still want them though.
Side note: I had no idea 80-pin SCSI was so overwhelmingly dominant. I never even saw that connector before I started browsing drives on eBay a few days ago. It's a real strain to find 68-pin and 50-pin drives.
Same deal with 7200rpm. I'm surprised how most of the drives on eBay are 10K or 15K. I don't want those speeds, they're too hot and frantic sounding. The 7200rpm drives I've had (now dead) were pleasant.
I wish I had stockpiled 7200rpm SCSI drives back when they were younger and more cheaply available.