You need a better information architecture to handle read replicas than just running a monolithic database. You need a much more complicated/limited one to do sharding.

I wonder if people see it as ripping the bandaid off to go straight to sharding. Or perhaps being multitenant introduces that idea. Seems like everyone, or at least the louder ones, discover that they have one 'whale' who gets to be too big even for a single database, or buys one of your other customers, and then where are you?