Sure, this is a more complex problem. My point is, for pkg-config files it is relatively easy to fix stuff that depends on non-standard files (I can write a devmanual section about that, but err... this is really trivial). The amount of these downstream pkg-config files is not as big as you might think (yet). So just saying "no" to all new downstream additions will not cause a big explosion and thousands of packages failing to build. Look at the tracker, it's just a few we know of. Cmake is mostly not affected, autotools is often complex enough to still find the libs, Makefiles need a one-line patch. And, by the time we discover more, we can work towards removing them.
It will raise awareness about this problem and about the fact that distros like debian tend to do it the lazy way. So it is a pretty easy way to improve portability across distros and so on. We shouldn't do things wrong just because they didn't blow up in our face yet. I am confused why this gets so much attention. The additional workload is really minor. So, not allowing this makes sense to me. For exotic exceptions and corner cases, we can still bend the rules. But, "debian does it too" is not one.
