Chromium, like any other Google product is not a simple build process. I think it is on purpose (my opinion).
That is why being a port maintainer is a thankless job: users all want the latest version, but it takes time and effort to get there and when you do, you may already be behind again
Your first sentence, that is a reasonable idea, but I think the complete thought should be along the lines of:
"Python27 has gone end of life, Python38 replaces it. If I set DONT_USE_PYTHON27, what can't build? Out of the ports that can't build, which ones do I have installed? Out of the ones I have installed, which ones do I use, which ones can I get rid of?"
Any system gets cruft, garbage on it when used for a while. You try an application, it's dependencies get installed, you don't like it, but then you never remove it. You wind up with extra stuff on your system that you forget about until you do a complete upgrade/reinstall and you install fewer applications.
Kind of like moving your house every 5 years, you clean the attic, garage, basement and throw stuff away. If you wait 10 years, you have more to throw away.