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More maintenance overhead. Supporting two databases takes more work than supporting one, and it’s unlikely that all current Open edX sites would promptly switch from MySQL to PostgreSQL once the support is added.
Operational unknowns. We collectively have a lot of experience with running MySQL at scale and have tweaked various indexes and queries to accommodate MySQL (and occasionally Aurora quirks). While it’s clear that Postgres can also scale, we may see performance regressions during the process of migrating any large site.