The interesting thing about the MK argument is that EPCOT's most successful period ever was when it was essentially nothing like the MK. It eventually became outdated and attendance declined, but as they've tried to make it a bit more like the MK over the years with IP, it's never really made a huge difference or brought back the popularity it had in the early years, when it was almost equal with MK in attendance.
I think Disney is better off with the parks being distinct from one another -- but they all need enough to do. MK is a multi-day park right now if people want to do everything it offers; EPCOT is the only other park that's potentially in that category and even that feels like a stretch.
Oddly enough, while I am predicting eventual problems for the MK as it becomes more difficult to get the headliners accomplished, while simultaneously losing decompression zones, Epcot is the one likely to be the beneficiary. The, "You don't need to buy Genie/whatever they are calling it now because it isn't necessary at Epcot," will be the motivation for the normies to choose it, if they want to check WDW out. Ticket prices will be cheaper than MK, photo in front of the big ball, solid nighttime entertainment without feeling like a sardine, can still meet Mickey. Can manage to get on most things, in a 'theme park reasonable' period of time, options for those that want the more intense option (Guardians, TT, Soarin) or a less frenetic option (American Adventure, Living with the Land, etc.), with the overlaps of Spaceship Earth, Frozen, Ratatouille.
What would really do it, is when everyone stops telling people, "The MK is the best bang for your buck, for ride variety, LL availability and ease of touring" and pivots to, "My least stressful day was surprisingly Epcot." The most irreplaceable part of MK is probably HM/PotC but given the increasing use of "problematic" regarding those two, by the time all these MK changes are completed, and the stress intensified, younger parents may no longer feel they are missing out. But I don't trust Disney not to gut Epcot even more between now and then.
