“very, very bullish on the theme parks” is a phrase Iger used recently and he also said he saw value in expansion due to the "extraordinary" demand for a Disney vacation. I'm not going to take him at his word since he was talking to Wall Street on an earnings call so it was all about trying to get the stock up, but this combined with the proposals for expanding Magic Kingdom could lead to more expansion should that get funding. Of course it would be a crime if MK got TRON, a PatF overlay of Splash AND an expansion of Frontierland before getting anything new in DAK or DHS, but alas, I wouldn't be surprised.
I also disagree with your thesis that increasing capacity decreases the value of Genie+/LL. Disneyland has a ton of rides and a ton of capacity, yet when I was in Anaheim last year, I didn't feel like I needed Genie+ in DCA at all, but absolutely felt like I needed it in DL. More to do in the busiest parks in the world still mean busy rides. The more rides, the harder it is to fit them all in, especially if you are doing a shorter trip or splitting between Disney and Universal. I find it super easy to get things done at DHS, DAK and Epcot without Genie+. The only hassle is GotG since there's the VQ. MK is where if I was on vacation, i'd get Genie+ for a day since it can be so busy there and there's a lot to do.
Last month, we went and chose to forego Genie+. I did get it for Epcot so as to get a Ratatouille Lightning Lane and then for Soarin' and I cannot remember if I got another one. While it was nice to have the LL for Ratatouille and Soarin', it wasn't necessary at all. We did not use Genie+ for MK but that was because I did not want to: A) Get up at 7:00 AM to book a ride and B) Stay on my phone trying to coordinate the next ride. We did standby (and the park was quite busy) and it wasn't too bad. I just didn't want to spend the amount per person to get the two or three rides per day. I think that Genie+ needs some work to actually become useful and I also think that it's existence is a revenue stream and not a benefit but that isn't a mind-blowing opinion.
I am in the mindset that expansion needs to occur at DHS primarily and Animal Kingdom (but within the scope of the park). Build out those two parks with rides, not just minilands, and that will spread the crowds out more and bring in more guests. I am not sure that Genie+ will become more or less used if you increase capacity. I think that Individual LL usage might increase but people are already using Genie+ a great deal already.
Ironically, at Epcot, I used a BG to ride Guardians and then purchased an Individual LL to ride it again. I kind of thought that it was worth it because we really enjoyed the ride and we only go once a year.