[The_Colonel]

I would actually put it exactly the other way round.

If you don't have a crystallized architecture yet, prototyping with a monolith is faster. You don't have to make the cuts (this belongs to this service) which are expensive to change later (unlike moving classes between modules).

Doing internal Java/.NET/whatever interfaces is easier than introducing external API (and associated infrastructure complexity, authorization, routing...), they are much easier to tweak. You have transactions, don't have to deal with a lot of asynchronicity, network overhead etc. For a prototype, I'd always rather do a monolith.