I'm all about avoiding premature optimization, and its fine to start with a classic postgres. But please don't cling to that - if you see MVP success and you actually have a reasonable chance of getting to >1mill users (ie, a successful B2C product) please please dont wait to refactor your datastore to a more scalable solution. You will pay dearly if you wait too long. Absolutist advice serves noone well here - it really does depend on what your goals are as a company.
10+ years of dev work for a few hundred tables worries me a lot. My last conversion was about 20 years of data across a few hundred tables and we did two-way data synchronization across DB products with about 1 month of work, with 2 devs. We kept the sync running for over a year in production because we didn't want to force users over to the new system in a big hurry. We only stopped because the license on the old DB product finally expired and nobody wanted to pay for it anymore.