Judging by the number of banking and government institutions that have weekly or nightly planned outages, there's appears to be a number of legacy systems where companies certainly think upgrades mandate outages. Even a non-zero probability of an outage is a cost.

Obviously this is fixable, but it's no free lunch.

The problem is that a lot of these systems aren't legacy. They've been built within the past 5 years, but the customers would almost rather continue on with their weekly outages than apply basic modern architectural designs that allow for functionality like round-robining updates behind a LB.