argv0's completely right. And I know a lot of "newbie" users of mIRC who had very little trouble picking up mIRC and using it. Usually the trouble they have is related to figuring out IRC commands, not in how to use mIRC itself.
I think the idea of "modern" may be more related to how it looks from a GUI perspective. Sure, it looks a bit dated right now if you don't add themes or other scripting changes that affect the GUI. That doesn't make it less modern. People are so hung up on making things "cute" rather than functional. Functional is much more important than making something look fancy. Just because Microsoft decided that looking fancy was so important that they changed many of there programs along with the OS so that it looked fancier doesn't mean that all programs need to do so. That isn't what makes something modern.
Besides, IRC itself isn't exactly modern. IRC is a text based chatting platform that has changed very little since it first came out. It will probably never become more than a text based chatting platform. mIRC can't make IRC what it is not. Sure, you can add fancy stuff to it such as emoticons and mp3 players and whatever else, but that doesn't affect IRC itself... no one else sees that stuff unless they also want to use such scripts. IRC is and probably will always be just a text based chatting platform and you aren't going to be able to "modernize" mIRC and change what IRC is.