I wonder why no one ever talks about architectures in the middle between those two - modular monoliths. The point in time where you're splitting your codebase up in modules (or maybe are a proponent of hexagonal architecture and have designed it that way from the beginning), leading to being able to put functionality behind feature flags. That way, you can still run it either as a single instance monolith, or a set o…
There is a more serious downside that you don’t mention: splitting things into modules takes time and involves making decisions you likely don’t know the answer to. When starting a new product, the most important thing is to get something up and running as quickly as possible so that people can try it and give you feedback. Based on the feedback you receive, you may realize that you need to build something quite diff…
Selling effectively is the most important factor, not speed. Speed is second as a factor (and obviously very important). That's actually what you're describing when you say success of a product is not correlated to how well it's engineered. It's correlated to how well you can sell what you have to the audience/customers you need. That's why some start-ups can even get jumpstarted without having a functional product via pre-product sign-ups and sales. Getting to selling as reasonably quickly as you can, in other words.
Go when you have something to sell. That's what the MVP is about.
Which also isn't the same as me saying that speed doesn't matter - it matters less than how well you sell. It's better to sell at a 10/10 skill level, and have your speed be 8/10, than vice versa (and that will rarely not be the case). Those are bound-together qualities as it pertains to success, so if you sell at 10/10 and your speed is 1/10, you're at a high risk of failure. Give on speed before you give on selling and don't give too much on either.