How do you assess the "rightness" of the platform before you even know the order of magnitude of, say, traffic or concurrent users?

I would love to rewrite my engine if it means I have a thousand times more customers than when I started. I mean, it's a win-win bet I'm willing to take: win now because I know the tools by heart, win later because I'll be making a ton of money later on and I'll be reingeneering a system I know by heart - which is so much cooler than writing fibonaccis in a language I don't know.

Chances are you're going to rewrite it all anyway for a strategic pivot, so why bother now with problems that you don't even know will ever exist? Just pick your best tool and make the most of it - premature optimization is also a form of procrastination.