I agree that a few people will be more reluctant to move on if an awesome new Simcity would come to market because they'd have to do without all the mods they got used to, but I'm sure 80% will move on immediately and the others would follow within months. After a while a program will come that will allow you to convert your BATs to the new version, and we will all happily play the new Simcity.
And, it is about time (IMHO) that Maxis releases its game code. Many games went before SC4, it's called abandonware! I've seen beautiful new versions of old games (like Kings Quest) based on abandoned versions of the original game.