While it's true that DL didn't add any meaningful capacity from 2008 to 2018, much of the attendance growth can be linked to a perfect storm of events in that period that wouldn't produce similar results in WDW.
2008 was right around the time that DLR started offering monthly payments on annual passes for local residents. This also coincided with the Great Recession, with 15 million residents of the greater southern California region who were looking for more-modest staycations rather than big-budget trips afar. This paired with an increase in the "coolness" of DL among the general population, alongside the rise of social media and the flaunting that goes along with it. The result was an attendance boom that bucked all traditional trends and conventional wisdom, with APs choking the resort at unexpected times: evenings in October are now more crowded than summer weekends.
It also didn't hurt that DL's sister park 100 yards across the Esplanade had a steady stream of additions (of varying degrees of quality) during that period, including an unprecedented relaunch of the park in 2012.
DL has always relied heavily on locals, but in the past 10 years the focus has shifted from locals who visit every couple years to regulars who visit several times per year. Instead of a world-class destination, in a lot of ways it has devolved into a local hangout spot, like shopping malls in the 80's and 90's. WDW, on the other hand, relies heavily on long-distance travelers who may only make the pilgrimage once or twice per generation; while these guests may not be as fickle regarding individual attractions and experiences, they're much more difficult to attract in the first place, particularly during a slow economy.
That said, I don't disagree with your conclusion that MK can and should add more capacity at a fairly modest cost (even with WDI's bloated budgets) to help distribute the crowds better. MK will always be the most-visited park in WDW; there's no reason that the park's capacity should be as low as it is. It simply needs more of everything to help absorb the crowds out of the walkways and into other facilities. The MyMagic+ system has only served to exacerbate the park's existing problems, while pulling away funding that could have been used to help mitigate them.