Lots of characters cause trouble in filenames. Shells hate whitespace for example. Terminals hate control characters. Other things get confused by commas, quotes, etc. Lots of shell utilities hate files starting with a hyphen, some with a plus. If you want to copy files to a Windows system or an MP3 player you need to avoid lots of things like question mark, star, less than, greater than, colon, dollar sign.
I guess my point is you can't design around everything which wouldn't like filenames to have specific characters. In fact it's kind of nice that the kernel doesn't know about the encoding system except that it won't produce single bytes that have the same ASCII value as / or NUL. That's kind of the bare minimum it needs to know to be able to handle entire paths and not just path components.
Sure I hate seeing filenames with crazy characters too. They usually appear to be accidently created. Or are the output of some terrible script which is trying to store too much information in filenames.
I don't think this qualifies as high-maintenance because most software pretty much seems to ignore it and treat filenames as sequences of bytes terminated by a NUL, and not do so very carefully with respect to whitespace or other unusual characters. I'd suggest that any kernel-level fix won't address enough of the issues to be a complete solution, and would require additional work in most applications and scripts to handle all filenames perfectly, just like now.