I would agree with your anology, however, ti can be turned around.
What if you have a car that can only do 40, when the limit is 70.
You would say "get another car". However, it's the same as saying "get another client", which, as we all know, people want to avoid.
I myself hate the idea of channel names that long, they are pathetic, memory wasting (server side, but just the client) and childish. But that is no excuse to not support them.
Yes, mIRC should have a limit, just the way the RFC states there is one.