>Good example is ETOPS - https://en.wikipedia.org/wiki/ETOPS - some time ago, we required flights over ocean to have three or four engines. Now, reliability improved a lot, and the engines got good enough that single-engine operation is not so catastrophic as before.

And yet, single-engine jets are prohibited from operating under Part 121 and must use Part 135 (the analogy here might be to an apartment building where the developer takes on the responsibility of structural code-compliance and safety, vs. a single-family home where the homeowner does).

There's a big difference in going from 4 to 2 engines, vs going from 2 to 1. And to be certified single engine jets must have battery backups, rapid depressurization tech, and a host of other things twin engines don't have to. In one case (Vision Jet) they use a whole-plane parachute to compensate for the lack of a backup powerplant