So, how do you choose that service level objective? How do you know which solutions to implement to not make things "overly reliable"? Isn't that more important question? As doing this without some sort of methodology will almost always result in useless solutions and overpaying to cloud and other hosting providers. Like implementing rather expensive failover within the datacenter, while ignoring how unreliable datacenters are and how cheaply you can implement failover between datacenters via DNS.

I like the idea of modelling availability/reliability for this. Even if you don't have the right numbers and do it on a napkin, not in code, it still can highlight solutions with best cost/benefit ratios.