In general I fully agree, YAGNI and all that, but from experience, mutable globals, unless used very sparingly, rigidify the codebase to a surprising degree and

In general I fully agree, YAGNI and all that, but from experience, mutable globals, unless used very sparingly, rigidify the codebase to a surprising degree and make it more error-prone, often for very little actual gain. As with any software engineering guideline, there are exceptions. E.g. Java-style loggers are so ubiqitous and "not that mutable", that it's generally good for them to be global. Edit: also from experience, suddenly wanting two of something where before there was only one, is not that uncommon.