Fly by wire
You say: "That is having an ever increasing effect on safety margins, and I wonder where it will all end?"
I am not saying I necessarily disagree, but where is your proof of this?
I haven't seen many (indeed any) instances of your assertion in the public domain.
The fact is that modern A320/B737 aircraft are generally very reliable, and hence, safe. That is why MPL is possible. The statistics show it to be safe, therefore regulators and insurers are prepared to take the "risk" - whatever that may be.