Besides argv0's excellent points, even if mIRC did add in some form of encryption similar to FiSH, people would *still* choose to use something else. I'd be willing to bet that over half of the people who use some form of encryption (FiSH or otherwise) would choose to continue using something other than what mIRC implements. So even among the users who want encryption, many wouldn't really be interested in mIRC's if it were offered. That means a significantly lower number of people who would want mIRC's version of encryption. You're probably looking at a total of a whole 1% of the total mIRC user base who would use mIRC's encryption if it were added. With that low a number, I have to agree with argv0 that it's going to be very low on Khaled's to-do list.
And just to stress argv0's point... just because you paid for mIRC does not give you any rights to demand anything from mIRC. For that matter, Khaled could choose to completely stop development today and you couldn't say a thing about it. You purchased the current version at the time of purchase and were given lifetime upgrades. That doesn't mean that Khaled has to do the upgrades that you want, though he might choose to if they are good suggestions. It just means that you can have free updates of whatever Khaled chooses to add/update/fix.
