The majority of IRC clients support it and have supported it for a very long time. mIRC has supported sending and receiving unicode text for a long time. People on older version don't even have to upgrade. They can just change their options to start sending text and receiving it in unicode. Other clients can also change their options without requiring upgrading to newer clients if the person doesn't want to. Obviously, there's a cutoff if you go back enough versions, but catering to people who use a 10 year old version of a client is not worth it.
In most cases, all it really takes is for channel ops/admins/owners/etc to choose to switch to a Unicode channel name (if needed) and ask their users to switch to unicode encoding. All it takes from users is to change their settings or upgrade.
The only time it will be more of a problem is if the server doesn't support unicode. Just because a server uses code pages doesn't mean it doesn't support unicode. But, for those that don't, then the users will be stuck on older clients or other clients until/if the server upgrades to support unicode.
The refusal to change is a common problem everywhere. That doesn't mean that those who are willing to accept change can't do so. Change doesn't have to cater to those who refuse to change. That may not be the nicest stance on the issue, but it's true nonetheless. If Khaled feels that unicode is a better option and wants to avoid continuing to confuse the issue as it was in the past when mIRC did support both options, then it's his choice. The download numbers support the fact that mIRC will remain very popular and that many people will end up switching to unicode just to keep using the newest version of mIRC. For anyone who doesn't want to use unicode, there's nothing wrong with 6.35.