I think you are being a little pessimistic here

It is called experience :-) As I mentioned, mIRC already needs to check for specific strings in certain numerics due to those numerics being used for different features on different ircds. These have had to be updated several times already. Unfortunately, it takes a long time for someone to notice that mIRC's behaviour has changed due to an ircd changing wording in a numeric - especially if the change in behaviour is subtle. In the meantime, mIRC would actually be doing the wrong thing in such a situation.

Depending on words in a server message along with the order of server events is not a reliable, long-term solution.