A lot of 0-downtime migrations end up turning into a months-long ordeal and can still fail.
Allowing maintenance windows means you can do things in a much simple manner. Need to take a snapshot of a DB without running into issues with a production system adding data? Sure, go ahead, you just saved two months.
The principle of something like zero downtime or switching a vendor out to save 20% may not stack up against the reality in opportunity costs, employee burnout, and customer turnover.
My company is reorganizing and so everyone is trying to settle old philosophical debates during The Churn. But everyone is overextended and a number of us have started pushing back. Just the meetings to discuss changing vendors might push the break-even point out 6 months, before you even get into implementation costs. The more time we spend working on infrastructure instead of customer-facing features and bugs, the more likely those customers are to wander off.
Allowing maintenance windows means you can do things in a much simple manner. Need to take a snapshot of a DB without running into issues with a production system adding data? Sure, go ahead, you just saved two months.