That bothers me too, but I'm starting to think that manufacturers are deliberately avoiding a read-only failure mode for security reasons: if your drive enters a permanent read-only state, how do you erase it before recycling? I imagine having used crypto from day 0 would be your only safeguard at that point, but even good crypto gets broken eventually, so how do you safeguard the data on that read-only drive in the long term? Is physical destruction the only answer?
On the other hand, maybe the total-failure mode that current SSDs enter is just a false sense of security. It's possible that the data on those chips is still available to someone who can bypass the controller. I don't have an easy way to check.