I know a startup that is struggling to replace SQL Server with MySQL. They are making this switch because the license costs became prohibitively expensive once they started scaling. It was not a problem in the early stages when the traffic was not that high. I think that's because the license cost is connected to the number of cores in the machine you are running it on.
Completely agree with @JanVladimirMostert on starting small and the need to re-architect sometime down the line.