Rather than brick the battery, if it is parked and below a certain level, release a normally open relay that completely disconnects the BMS from the cells. Recovering from this state would require special charging and recalibrating of the BMS (possibly done at a shop) but it would be better than bricking the pack.
Disconnecting the BMS does not eliminate self-discharge, only the parasitic load. Draining your batteries to the point of being functionally dead, and then leaving them in that state for an extended period, will still "brick" them just as badly.
Switch does not solve the problem, "backup battery"? You're just delaying the inevitable, why not just demand that they make the batteries bigger in the first place?
What good does a "mini engine" do? You can't turn it on automatically, that's an enormous safety risk and would never make it past insurers or the government, and it can only last as long as its fuel holds out, again you're just delaying the inevitable. Solar panels don't help if you're parked in a garage, which is where most people park their fancy roadsters, and they'd have to be huge to maintain the charge anyway.
And absolutely none of this contradicts my original point: It's a technological limitation. This is how batteries work. You're not proposing a solution, you're proposing unworkable band-aids.
Rather than brick the battery, if it is parked and below a certain level, release a normally open relay that completely disconnects the BMS from the cells. Recovering from this state would require special charging and recalibrating of the BMS (possibly done at a shop) but it would be better than bricking the pack.