I have a thought, coming from Raspberry Pis that use a microSD card as their main disk. People would report that heavy usage of an SD card would wear it out, particularly writes. Does Steam or the OS do any writing to the disks while playing from them? If so, unless you’re downloading new games, mounting in read-only mode might extend their lives a bit. Again, just a thought and maybe it’s not an issue since it’s primarily reads being done, or maybe it’s crossed your mind already.
Depends on the game, but there’s going to be written sectors no matter what I would think. From Save games to generating terrain in things like Minecraft. It would be interesting if they could do a paired folder thing where all the writing would be done to the OS drive and read from the SD.
That being said, I think the wear will be much less than on the RPi since it’s not running the actual OS off of it, not that I have anything to back up that theory…