You're going to run a production system with a bus number of 1?

I think you mean a small fraction of 3 engineers. And small fractions aren't that small.

So far I have seen a lot more production systems with a bus factor of zero than production systems with a bus factor greater one.

The cost being a fraction of 1 does not imply it's one person. 3 people each spending 2 weeks a year on the service is still a fraction of 1.

It is three opportunity costs. No free lunches.

Nobody implied it was free. Yes there are opportunity costs, and they add up to less than one sysadmin of opportunity.