Final Fantasy's success in 1987 was unexpected, so if FF II had flopped, it would probably have been recorded as the so-and-so sequel of a good game. But we wouldn't be here discussing it, so we wouldn't miss the series because there would be nothing to miss.
Incidentally, there's a chance that the Dragon Quest series would have been hyped a lot more in the States, and now we would all be DQ fanboys in a DQ forum.