Well, that's certain. I don't recall that anybody uttered the magic words -- "There's no way he could have survived that!" -- which would absolutely guarantee that he would be back... but in this case I think the magic words were implicit.

As for other supers, you can always assert that (a) Syndrome missed some of the public supers; (b) there were an arbitrary number of covert supers that Syndrome never knew about; and/or (c) new supers have been born/had laboratory accidents/picked up oddly glowing mystical artifacts/been exposed to weird radiation/dedicated themselves to revenge on all criminals since the older generation went into hiding.