It's usually the company leadership that can't tolerate it to have downtime.
And it's fact, they are hit an unplanned with downtime of one service or the other every month or so because of an outage. They are used to it.
So if you plan for it, explain it, and limit the scope and time of it, it usually goes very well unless you are a fortune 500, a hospital or something alike.
I once caused a production outage in a retail company that caused all cash registers to stop working. The team that worked on that had pushed in a last minute change and didn't test if it handed going offline gracefully.
Right now I'm on call for an identity provider used by people in various timezones, including logistics workers in places that operate 24/7. Even when we do weekend upgrades, we still cause quite a bit of collateral damage. 10 minutes of time, multiply by the number of employees affected. It adds up fast.
Indeed, but who cares about the system design anymore? How many companies/ teams can claim honestly they even had a person with proper DBA competency, while features over features were added in sprints doing the minimum required to get the feature shipped out at the soonest possible (usually one DB schema change with feature and then one or more to add index due to performance regressions)? DBA competency is only sought when DB schema has fubar'd to an extent that frequent outages are norms or the version used is EOL'd by a few months at least. And by that time the people who "designed" the system are gone, not having documented ever why a given decision was made.
And that is how my friend...
DB upgrades are hard.