This isn't an either/or situation. All of WDW is painfully underbuilt for the level of crowds that it gets these days (and has gotten for years). MK needs more attractions to soak up the crowds, and the other parks desperately need more attractions to soak up and distribute the crowds more evenly.
If in doubt, just look at wait times for similar attraction in MK and DL; DL's will almost always be shorter (often considerably shorter) despite relatively similar crowd levels (MK's annual attendance is skewed by it's constant upcharge "parties") because DL simply has more to do. At MK, nearly anything that could be considered an "attraction" (a term that WDW plays fast-and-loose with) boasts wait times at least 30-45 minutes. At DL, only the big E-tickets demand those sorts of waits.
Yes, the other 3 parks are in even worse shape. But that doesn't mean that MK is where it needs to be.