Besides which, anyone with the programming skills can write their own IRC client anyway. It's not like mIRC is the *only* Windows chat client...

Yes, However mIRC does have a unique scripting language, And it is the *only* chat client to have that. With access to the source, Advanced users could maybe gain more insight into how the scripting engine works... Expand it a litte?

mIRC could be developed for other Operating Systems, And long after Khaled quits development mIRC's growth would continue, Thanks to open source.

There are advantages to open source. But the time for it isn't now.