We frame these situations under the banner of "solve the problem you have".

Avoid premature optimisation for problems you might have. If your site only has 1000 users, there's no benefit to deploying it in infra scaled for a million users. Could you have a million users in the future? Sure, solve it when you start heading that way. Could your site stagnate with 20,000 users? Absolutely. It's a waste of engineering efforts (and money) to scale for 1 million users now.