I don't think "later gens always beat new older gens" is a good argument. The reason why later gens have been doing better is because of the increased marketspace for K-pop around the world: gen 1 was primarily in South Korea, gen 2 expanded into Japan, China, and SEA, and gen 3 had a global boom into the rest of the world. However, if K-pop is decreasing in popularity in the space where gen 3 groups operated, then it stands to reason that gen 4 may not do as well.
Gen 3 isn't special: they just happened to pop in at the right time. I guess what "international heights" means is open to interpretation though. SEA/Japan/China are still "international," and there's no reason why a gen 4 group couldn't end up more popular in Japan or China than any gen 2 or gen 3 group. But my interpretation of international was "the west" since that's where Blackpink, Twice, and other gen 3 groups notably came in.
Maybe OP can clarify?