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.

>making a single RDBMS server in a container

On what disk is the actual data written? How do you do backups, if you do?

In most setups like this, it’s going to be spinning rust with mdadm, and MySQL dumps that get created via cron and sent to another location.