hmm, but the abort command does say this
Code:abort = { owner = { or = { not = { religion_group = this } not = { num_of_cities = 1 } not = { vassal_of = this } } } }
oh tricky = sign...
That simply means that the mission fails if the target country no longer has one or more cities, ie is eliminated altogether. For some reasons '='-signs means "equal to or greater than", instead of the traditional meaning while comparing numbers.
For example. prestige=THIS doesn't only trigger when both countries coincidentally is exactly as prestigious - or it would practically never happen. It simply compares which is the more prestigious.