Firstly realize that many of your assumptions here are false:
The rendering problems are with other font sizes in excelsior.
No, the rendering problems exist at the recommended setting. It's a very visible halo (to me), and this is something the author himself admits (it's in the FAQ), though the severity of the issue is subjective.
Even when a font has a perfectly straight line that matches up with screen pixels, CT [ClearType] will create a slight, tiny distortion to its right or left. It becomes more noticeable when the text is brightly colored. This spoils the precision of Fixedsys even though it is a rather minor effect.
He claims it's minor, but I would purport its way too major for me to even use. I know others who feel the same way.
I'd rather mIRC change the default to any *standard* unicode font that is packaged with Windows; not replace Fixedsys with a lower quality variant just to get some unicode characters.
But again, the "default" font isn't very relevant here. Unicode isn't even enabled by default in mIRC, so if anything you should be complaining about that-- not the font that's used. If Excelsior was indeed enabled, it wouldn't matter with unicode off.
And tbh, 1 € is now 1,5 $. so people needs € as a marker what currency is the worlds leading currency.
Er, the euro is not the world's leading currency, sorry.
1 Euro = 1.4576 U.S. dollars
1 British pound = 1.9517 U.S. dollars
If anything, it's the pound... and there is a British Pound sign in the relevant ANSI codepages, so once again, problem averted. Of course, this discussion isn't about Unicode vs. ANSI, it's about fonts, so I'll leave that one be.
mIRC using still older format not suporting most languages is not good or is mIRC made only for americans?.
#1. mIRC is certainly not made for Americans. Proof: Khaled isn't American.. he's actually far from it. What's more is that Khaled implemented SJIS/JIS conversion (for the Kanji and other Asian fontsets) even before Unicode; a specific implementation for a specific set of languages, that means he does indeed tailor the application to other languages.
#2. If the "older" format you're talking about is using ASCII codepages, then yes, mIRC is still using it. It's still using it because *people* are. This is also a reason you can't just turn on unicode and assume everyone will be happy, because a lot of people are still using single byte encodings.
Again, the conclusion is this:
If you want need to use the euro sign in every day talk when you compare prices, go ahead and change your font / enable unicode. No one is stopping you. Your friends should still be able to see your text without changing their fonts from Fixedsys (again, font linking.) But does your usage justify making it default? Probably not.
Eventually, mIRC will have unicode on by default I would hope. When that is, I don't know-- Khaled might just be working on it right now... But until that day comes, its useless to have a unicode font by default when unicode isn't even on by default. And when that day comes, the font should certainly not be a custom font that renders poorly in the operating system for users with LCD monitors, especially when the rendering can make it completely unusable for many. I'd rather see an ugly font that works for absolutely everyone over a nice font that only works for "most".