FF2 is a very important game in the series for several reasons:
1. It introduced chocobos and Cid for the first time.
2. It was the first game in the series to feature named main characters with storylines and basic personalities. Though they went back to generic heroes in FF3, it was for the last time before they switched back to FF2's style for good.
3. It had a complex battle system for its time as well as introducing temporary characters and allowing you to fully customise your characters, which you could not do in the original.
I agree that if FF2 had flopped we would not be experiencing FF today, and I think the JRPG genre as a whole would be quite different. One good thing that would have come out of it is no Compilation of FF7 and no Spirits Within. I still want those 2 hours of my life back.