I know it sounds bad, but in practice, it really did work fine for us for quite a while.
1. We didn't experience that many incidents that couldn't wait until working hours.
2. There was never an explicit expectation to keep an eye out. We did it anyway because we were at an early-stage startup, and we all deeply cared about making our products work for our customers.
I know this from a few startups and it really is not that bad. You really triage what should wake you up and what's ok until morning. It works well as long as the technical founder is ok playing a goalie and essentially being always on call (even though others catch a lot of alarms).
It stops working when the company grows and no one understands the whole system and you need on-calls from several teams. Then the company does some formal on call rotation and it's fine again. It hurts during the transition only.
1. We didn't experience that many incidents that couldn't wait until working hours.
2. There was never an explicit expectation to keep an eye out. We did it anyway because we were at an early-stage startup, and we all deeply cared about making our products work for our customers.