Proper engineering isn't that much of a concern when you have 0 customers, and by the time you have some it's too late to change.

Besides nobody is claiming that it's impossible to build a successful products with dynamic typing. It's just not as good. You can build a successful product with zero comments in your codebase, doesn't mean it's a good idea.

> It's just not as good

Again, the evidence (as limited as it is) suggests otherwise. You are more likely to succeed if you're going with dynamic language and not doing "proper engineering". This has been widely accepted before type-checker era, and I see no reason why it would be different now. Utilize type checker when it's free, but don't waste time on type puzzles.

"Proper engineering" doesn't get you to product-market fit faster. All it does is tickle your ego.