It's easy to say this, but in the real world, most of the critical path is heavily-dependent on third party integrations. User auth, storage, logging, etc. Even if you're somewhat-resilient against failures (i.e. you can live without logging and your app doesn't hard fail), it's still potentially going to cripple your service. And even if your entire app is resilient and doesn't fail, there are still bound to be tons of integrations that will limit functionality, or make the app appear broken in some way to users.

The reason third-party things are in the critical path is because most of the time, they are still more reliable than self-hosting everything; because they're cheaper than anything you can engineer in-house; because no app is an island.

It's been decades since I worked on something that was completely isolated from external integrations. We do the best we can with redundancy, fault tolerance, auto-recovery, and balance that with cost and engineering time.

If you think this is bad, take a look at the uptime of complicated systems that are 100% self-hosted. Without a Fortune 500 level IT staff, you can't beat AWS's uptime.