As with so many opinions these days, it's at one extreme and ignores the middle ground.

For some apps, the big issue is when you schedule the downtime to happen. If you are truly customer-centric, you should schedule for a time that causes the least disruption for your users. Of course, a decision like this depends on other factors - the number of users likely to be affected, how critical the app changes are, costs, etc.

I've noticed a number of companies in my country taking sites and apps offline in the middle of the workday when they're very likely to be in use. To me, this is unacceptable - the work on the app should be done after hours.