I actually disagree with you here. There are costs above and beyond the engineer's effect on the balance sheet. There's the partial salary of management to manage them, plus asking them to document their work and train others so that the database won't have a bus factor of 1. So in well-run engineering departments, there's no such thing as paying for a "single" engineering salary. You have teams; a team maintains the system and it has a pre-existing workload.

A large part of the value of popular platforms is precisely that they are not bespoke. You can hire engineers with MySQL/Postgres experience. You cannot hire engineers who already have experience with your bespoke systems.