I get what you're saying, however look how fast MMRR has gone up. They started on that last year and it looks to be on pace to open potentially next Summer. The Spider-Man ride also didn't take long to build either by Disney standards.
Also, Avengers is being built on what is now a parking lot, so there's not any demolition that needs to take place and since that's all backstage in a self contained area, the construction shouldn't interfere with daily ops much at all either.
Absolutely agree about the construction aspect. The great thing about the potential expansion of this land is that the main pad is already basically ready to go. I was just looking at the overhead on Google Maps (which has been updated!) and I didn't even realize that the trolley roundhouse is directly behind the building with the Quinjet, so it's tracks run right through. I guess if they do expand that direction, the roundhouse will need to be moved and the tracks re-routed. I'd even be ok with them getting rid of the trolley all together. I never really thought it was a great addition.
This is way down the line thinking but I wouldn't mind seeing Marvel eat up the rest of the Hollywoodland area. It's always been one of the more troubled sections of the park and one of the few areas left in DCA that is kind of stagnant and weird. That rumor has come and gone a lot but it's not like Marvel doesn't have the staying power and variety to fill out that entire area.