Well, yes and no. If you already have your database running in production with many transactions and live connections 24/7, changes to the schema might not be hard per se but always needs careful planning and execution. Additionally, in Postgres a change like adding a column locks the whole table (although I hear this is going to change) so writes are off for a short time. If this becomes unacceptable, you go for a blue/green deployment which has its own gotchas.
So, while I agree with your main point - there is nothing in Postgres that prevents rapid prototyping, and I would chose it over Mongo any moment, I understand why some people might prefer the more "dirty" approach.