The problem with single instance is that while performance-wise it's best (at least on bare metal), there comes a moment when you simply have too much data and one machine can't handle. Your your scenario, it may never come up, but many organizations face this problem sooner or later.
I agree, my point is that clusters are overused. Most applications simply don't need them and it results in a lot of waste. Much of this has to do with engineers being tasked with an assortment of roles these days, so they obviously opt for the solution where a database and upgrades are managed for them. I've just found that managing a single containers upgrades aren't that big of an issue.
