[jsingleton]

As with any engineering problem, there is usually a trade-off involved. Premature optimization is bad but software does need to be fast enough to start with and flexible enough to be improved later.

You don't want to code yourself into a corner by accident. It's fine to knowingly take on technical debt, as long as you have a plan to fix it in the future. Even if this is never required.

I always try to take a pragmatic approach, as things are usually not as binary as these simple rules of thumb assume. The real world is typically very nuanced, which is why engineering can sometimes seem like more of an art than a science, and why experience is so valuable.

Shameless plug - I hope this attitude comes across in my book that focuses on web application performance issues: "ASP.NET Core 1.0 High Performance" (https://unop.uk/book/).