I’m curious what services you interact with that have five nines availability. Most services that I interact with, AWS APIs as an example, top out at an SLA of four nines for availability.
Which is to say, I would not be surprised if many APIs with a four-nines SLA were actually closer to five nines.
From a planning perspective, you should probably stick to the public guarantees (i.e. the SLA), though. Although things can always go horribly wrong, like in the big Atlassian deletion snafu recently.
There really isn't much to discuss in the industry of SRE. You basically have to be flawless. Its like the janitor. Miss the garbage pickup one day and heads roll. If you have SRE's and the service is down you are done. There is no longer any nuance which is why the CEO guy in the OP doesn't think about SRE anymore than the garbage collection.