[t0mas88]

These are exactly the things to worry about in an application that has enough scale for it. My usual approach is to have a wiki page or document to describe these limitations and roughly the order of magnitude where you will encounter them. Then do nothing and let them be until that scale is on the horizon.

There is no point fixing a "this could be slow if we have more than 65535 users" if you currently have 100 users.

I usually add a few pointers to the document on how to increase the scaling limit a bit without major rebuilding (e.g. make this cache size 2x larger). Those are useful as a short term solution during the time needed to build the real next version.