[bux93]

I think everybody has a horror story about code depending on the empty string and NULL being the same, silly stuff like that can trip up migrations.

The main reason I'd say is that there's no functional benefit to ripping out a database and replacing it, so there's always something more important to do that actually drives revenue.

I'd argue that postgres brings with it substantially lower risks in terms of license compliance/audits/price hikes. Not sure if that can drive a migration, but it should be reason enough to select open source for new projects.