Problem is that RDS comes at a price. It is purely about operation cost.

When you have 1500+ databases these cost add up. At that point, this kind of techniques are required to self host the databases. Price per DB with HPA na VPA is way lower than what you would pay for managed databases as well as you can hire a full time devops+dbadmin and still be cheaper.

This is a perfect example of a cost savings opportunity that offers no benefit and is really just a foot gun when it comes to operational complexity. The cloud cost savings almost never justify the operational costs here. There’s typically a very long list of other cost optimizations to be made before this would ever be on the table for me. If you’re talking about 1500+ databases and think a single full time devops+dbadmin in some kind of unicorn person is going to be a better option, I feel bad for both of you.

Except the typical cloud prices are dialed for USA, and it's not that unusual in my experience for the cloud savings to be high enough to add more than one highly skilled engineer who will give large positive impact all around.

I am not implying hiring single full time dbadmin, you can hire a team of them and still be cheaper that what AWS would cost at that scale.