What does party bonus being removed mean?

Well to be fair people already kinda do that by almost everyone using the "good" training place, I would say that the only ones that do explore/go on adventures are those that hate crowded areas due to ks or other stuff and questers since if you look at what most people do is train in an area till they're high enough for the next one and repeat.

I guess that after this areas/monsters that don't involve a quest will be removed since people will just spam outside the park J>PQ and not even care to look for a good training spot that isn't crowded.

Poor design != intended feature.
Just because players do it doesn't mean Nexon cannot change it. In fact, they did, and they did well in Big Bang.