One thing I kept coming back to: your "how will the system grow?" question assumes you can answer it. For an established business, sure. But for a brand-new project with no users yet, that's just a guess.

So maybe the earlier question isn't "which architecture fits my requirements" but "how much do I actually know about my requirements yet." If the answer is "not much," the simplest, cheapest-to-reverse option wins by default — not because it's technically better, but because you can't design for growth you haven't seen.

Curious if you think that's a real first step, or if it's actually fine to guess and adjust later.