Id say that you could never be sure what happens if something in time got changed.
Have any of you heard of the Chaos Theory and the butterfly effect?
Basically it states that if a butterfly flaps it's wings in, Lets say London, it could theoretically cause a Tornado etc. in Australia. Yes, I know it sounds kinda far-fetched but it sort or makes sense. Basically, Even the tiniest things have unforseeable consequences. So If the JME was destroyed it could cause the with sorceresses to take over the world, Squall never being born, SeeD never existing, you name it it could happen. Ultimecia could technically never have been born. If you try to think about what would happen if something in time was being changed you will end up with a lot of loops and endless cycles. it's best not to think about it it'll just give you a headache.
Good thread FE!! Really Interesting..... ^_^ *happy*