EXO didn't have a chance to fully dominate ever , at least not the way it is conceived now- they did dominate the way it was perceived before BTS got this popular and kpop were.more accepted in the West-, since the beginning. The 12-member group split in two 6-member groups was already a concept a lot of people found difficult or uninteresting to get into. So even if they were huge in Asia they would still find it difficult to break beyond the niche audience of kpop in the West.
Also they were mainly focused on Asia which also hinder their opportunities because SM was very much focused on the Asian market and Western success was pretty much something they seek but never truly, aggressively targeted like BTS and Bighit did.
And last but not least, nothing lasts forever. Even if the remained 12 with EXO-K and EXO-M fully promoting they would have already hit a slump by now.
And also other issues would have raised from their full line up staying together and controversy would have followed them around regardless. All those things can take a toll on any group.