Template meta programming is an entirely separate, incredibly complex, programming language distinct from c++. The only people I have ever seen use it in production are exceptionally skilled programmers. Everyone else flees and cowers. If they try to change it it will not compile. So why is this good? Because it only allows the framework to be extended in the ways intended by the original designer. Anyone wanting to update this unholy beast will be a master programmer who’s investing significant effort. That implies the updates will be good.

Good code is easy to change. Great code is near impossible to change, but easy to extend.