Also it seems that most Trojans out there, which are based on mIRC user 5.7 or earlier
You can hardly fault mIRC because someone took v5.7 and added it as a payload in a trojan. Ppl get trojans not by downloading v5.7 or any other version from an official mIRC site, they get them by opening every attachment sent to them by email, by downloading from strangers, by clicking on every url they see,by using backdoored scripts. Warnings about these things are largely ignored, as are telling ppl to keep windows and their virus scanners updated.
There are ppl who for various reasons want to use an older version of mIRC, and in some cases HAVE to. If they couldnt get those versions from an approved site, they would go hunting for them and who knows what they would end up with. I have one puter that just refuses to play nice consistently with any version after 5.41and since that puter is a 16bit machine, using v6.x wouldnt be possible.
Fact is, there are many reputable software programs out there with security issues, and more found every day. There are ppl who will exploit those, there are ppl who wont bother to install patches or upgrade even when they know they should.