Scaling issues can present themselves in numerous ways which may not require an in-memory DB, sharding/partitioning, hot/cold or such to be changed, they may even be already present.

In a lot of cases, these can be used and added without locking you out of migration since parts of these are deeper application level or just DB side. The query planner isn't the end-all of performance, there is plenty of differences between MySQL and PgSQL performance behaviour that might force you to switch even though the query planner won't drastically change things.