This is true about world gen needing to be more balanced. I believe the intent is to have civs that are capable of attacking with (if not literally showing, at least implying with a seemingly endless stream) multiple hundreds of entities if they so choose. Also, civs will no longer always be: generic human, generic dwarf, generic goblin, generic kobold, generic elf, the end. They'll be related directly to who actually, really exists within trading distance (which again will require balancing, since right now it's rare to find yourself near more than two civs at most by the end of world gen). this could mean angry, expansionist humans, or it could mean a civ comprised of a goblin/elven mix, or whatever. And maybe there are two and maybe there are seven. And maybe some are right in the middle of fighting each other.
It also means those civs will be capable of getting immigrants, just like your's (something like this has already been implemented in world gen when cities are abandoned, so it's getting close). That means you won't necessarily have to wait for the children of the corn down in goblin city - whose entire adult population has been decimated - to grow up before they start attacking you again. But you shouldn't expect to see very large parties anymore, and it's entirely possible you could destroy them all before they even have a
chance to breed in the first place, if your fortress/dwarfs end up being just that badass.
But even then, no worries. Because somewhere in your world there might possibly be a much larger goblin stronghold, ruled by a demon, and they may have just taken notice of your genocide attempt.