First off, there is nothing to suggest IM programs like MSN and AIM are stealing people from IRC, you're all basing it on your own experience which is ludricously inaccurate.
Secondly, IRC is never going to die, nothing geeky ever does, there's always someone, somewhere who wants to be "retro" and "hip".
Thirdly, incorporating MSN into mIRC would be a) absurd b) pointless and c) time-consuming.