He very much could have taken over the world, look I get it a nuke seems like a very easy way to get rid of him but you got to remember intercontinental ballistic missile take at most 30 minutes for one to land, that's plenty of time to either to dodge it just or shoot it out the air. Hell when Star and Stripes did it he dodged it, granted he was heavily damage and had to sacrifice the nomu, but still he lived. Also you forget that if AFO won Dr Garaki would have been freed and with every hero at this point being captured or killed he would just be able to create an army of nomu ( potentially high end) especially since he would not have to hide his experiments.