Oh hmm what do you mean by concurrent here? Several writes in succession to the same place or writes that other users need to be able to read quickly?

The majority of new things people build fail (say 95%), but it's still worth building for the success case because essentially all of the value comes in the success case. It increases cost on average but it means your website doesn't go down under load the second it's successful. This is only true to an extent of course -- don't build your startup like it's Google. But to a certain degree it's worthwhile architecting for some scale before you have it.

In the context of embedded things of course you should use something like SQLite as opposed to postgres, because there aren't going to suddenly be millions of people using your CO2 monitor (for example). But for web stuff you can plausibly have user counts that span 6 or 7 orders of magnitude, so you want to be prepared for that.