Look at it a different way, because I think you are correct that the bulk of the coin is made in MK. What if we could actually spread out the visitors in all four parks relatively evenly because people wanted to visit all of them for a full day or more? What if we could spend a chunk to make EPCOT appealing to younger kids because there was stuff for them to do other than walk really far, so that they were as interested in visiting it as they were in MK? I don't love IP for the sake of IP, but Ratatouille in France, that might work. Add something to the countries that already have ride pavilions. To me Mexico is just loud and brash and a little bit silly, but don't change it initially. Re-open Wonders of Life and reinvigorate Imagination - no buildings to build, so maybe not as expensive to do. All of sudden, we start to bring kids back to the park. Take Innoventions spaces, and give us a tour of Wall-E's ship, or something else futuristic, but with a relevant IP. I get that the old idea really won't work any longer. Now take 2-5,000 people per day out of MK because they have nothing else to do, and put the in EPCOT. Increase chance of selling something to them because they are seeing something different than MK merchandise. What if through a 7 attraction addition, you could create the same opportunity in AK. DHS seems to be going in the right direction, but they are just replacing things that have been removed, so I think they have a ways to go, and probably don't have the space to really add much. We are probably relegated to a 1/2 day to almost a full day with little need to repeat.
The goal should be for each park to offer a unique experience that makes people want to go there for more than a day. That makes the week long vacation worth it, and if there really is that much to do, there is more compulsion to come back year after year. That also creates more future revenue for the company, rather than ticking people off with the current "too expensive to stand in line". It also would work to spread people more evenly between at least 3 of the parks.
That is a lot of up front money, and I get that, but if they had spent the requisite $.5-1 billion per year keeping the parks up to date for the last 20 years, they wouldn't be in this boat. Glad that they've spent 3 billion in the past 5 years, but they are still behind by A LOT. This is why local parks add an attraction almost every year. You need to do that to keep people coming back. Disney can rotate that between the 4 in Orlando, but they need to be consistent.