I agree with you that C-ticket attractions are the greater priority for the tipping point problem. Disney has been exacerbating the issue with cuts meant to save money in the short run -- reducing parades, reducing entertainment. Reducing hours (spreading guests over more hours helps to reduce lines).
So my disagreement with you in minor -- A new e-ticket to MK may bring a few more guests, but it doesn't bring 15,000 new guests per day. What was the last new popular attraction at MK? Maybe the Mine Coaster... 2014... If you look at annual attendance at Magic Kingdom from 2010 to 2019, you see an upward trend every year, but no sudden spike connected to the Mine Coaster.
MK attendance by year:
2012: 17,536,000
2013: 18,588,000 (6% increase)
2014 -- Mine Coaster opens in May -- 19,332,000 (4% increase)
2015--20,492,000 -- (6% increase)
Then notably, attendance at MK was basically flat from 2015-2017 -- While you can argue the Fantasy Land expansion (starting in 2012) might have brought more guests, the most popular attraction, the Mine Coaster, just continued the trend. Then you had increased MK attendance in 2018-2019, despite really no new attractions of note.
Now, MK operates differently than the other parks, because of the number of attractions as you cited. In terms of number of "great" attractions... e-tickets, whatever you want to call them, the difference is even more stark.
Nothing noteworthy has been added since 2016, with Frozen Ever After. With Soarin and Test Track, you really only have 3 attractions that people feel they must-do. Leads to people spending limited time at Epcot, or even passing on Epcot entirely because "there is so little to do." (if you are going primarily for E-ticket type attractions).
Open Cosmic Rewind -- Yes, lots of guests will show up just for that. And where will those guests come from? AP holders for the newest attraction.. and some guests may convert a MK/DHS/AK day into an Epcot day. Because that's a big addition relative to the entire park. (But I don't think it creates a massive boost to out-of-town visitors.... rarely do people plan a trip because of 1 tide).
Next example -- Star Wars Land and ROTR -- And same can be said for Toy Story -- I said it's rare that people will plan a trip because of just 1 ride. But Star Wars has a huge base of hard core fanatics, and Toy Story is probably the most popular Pixar property over generations.
Now, compare it to a Tron Coaster -- I dare say most people under 45 don't really have much of a memory of Tron. The more recent remake wasn't a huge hit. People aren't going to rush out of their homes for Tron branding. In a park that already has 3-4 coasters (depending what you define as a coaster), it adds another. People that love coasters will want to try it... they will add it to their itinerary along with BTM, SM, etc.. AP holders may stop by for an extra day to try it. But I don't think you'll have lots of people saying, "oh.. I was planning on skipping Magic Kingdom.. but now that they have the Tron Coaster, I'll go."
One of the big mistakes of FP+ was to promise every guest 3 FPs, but then not have enough attractions to really warrant a FP. (less a problem at MK, more a problem at the other parks).
Basically... long way of saying, anything that increases capacity is a good thing. I agree the need for C-ticket items, parades, shows, etc... is greatest. But adding e-ticket is not a *bad* thing either.
So even if an E-Ticket doesn't bring in 15K more per day.
What if it brought in 2K more?
At any one time, the queue and the ride will be occupying 500 of those 2K.
Where will the other 1.5K go?