Site reliability basically boils down to two things: Whether you build it right, and whether you actually spend enough on hosting (and on the right things). I have a client I've billed well into the 5 digits in pounds over the last year due to incidents that would never have occurred if they'd been willing to invest half of that in hardware upgrades, for example.

So I agree: It's so much down to priorities, and basically if your pager goes off all the time, you've likely prioritised wrong - it's false economy. Adding an extra server to get the capacity needed to prevent a total meltdown is likely going to be worth far less to you than the time and lost business those meltdowns are. And thankfully when you do things for yourself you get to make those choices.