If I'm mistaken about something, I would appreciate clarification. Is my mistake using box-drawing characters? Or is it thinking that they're disliked?

I've seen different opinions on the topic. I'm not sure there is anything single opinion of the Unicode Consortium here, but I'm not a member of the Consortium.

What we can see is that these characters have become very popular and useful, so it doesn't really matter whether the original intent was to move these things to a higher level protocol. Today they are here, and they are useful.

There was a discussion on the mailing list some time ago when there was a suggestion to add codes for underline, bold, italics etc. I can tell you that that is not a very popular idea.

Sometimes characters were added to Unicode just because they already existed in another character set. The idea is that Unicode should support lossless round-trip conversions to any other character set. Box drawing characters were part of the DOS character sets that were carried over to Windows.