Demanding the support of spaces in filenames significantly complicates code as simple space delimination no longer works and other delimination schemes are much more error prone -- forgetting balancing quotes, any one? While you are allowing spaces, you probabaly are allowing all possible code points or maybe even a null byte? Thinking about it gives me headaches.

I hate hearing people using 21st century or modern as reasons for inflating complexities. Without rein on complexity (whether it is hidden or not), the future is doomed, whatever you are building. While I am not saying we should avoid complexity at all cost, I am insisting that all complexity should be balanced with merits.

The merits of filenames with spaces is they read better in a GUI explorer. Whether that merit balances out all the complexity it brings is individual dependent. For me, that merit ranks very low and I avoid spaces in my filenames at all opportunities. For some, they need those filenames to be readable. And there are solutions. One solution, from those who don't code, seems to demand ("developers", paid or not) that every tool that deal with files should handle this additional complexity, regardless of their context and what they think. Another solution would be to add an additional pre-step of copying/renaming/linking/aliasing. With the latter solution, the complexity is confined.

I guess for some, it only matters with "I do work" or "they do work" rather than the big picture. That is fine. However given the context of you are working with Makefiles, then you are a developer at some level, you are supposed to do some work.