Everything you said is true... but the amount of energy that will be used and the pollution that is generated producing that electricity is still unknown.
When the system is running, how many buses will be eliminated? That is an unknown, it is possible Disney will only eliminate a handful of buses. I don't know how many buses they currently have running from those hotels to EPCOT or Studios and since those are the only parks they will be serving then you would still have all the buses running from those parks to MK and AK... And if the gondola system has to shut down when there is lightning in the area you would still have to be able to pull up buses when storms hit or people would just be kept from the 2 parks which would cause all sorts of complaints. It is entirely possible that once the gondolas go into service that the total pollution generated by Disney will be greater than it is now simply because they don't eliminate enough buses.
Well... since you liked doing round estimates before... you can ballpark what the minimum number of buses are needed for a route by estimating their cycle time, load, and unload. Without even running the numbers, I’d bet you’d find a single route that takes 10mins on the road would take at least 3 buses to maintain a 15-20min cycle. Then look at how many routes are impacted... and you at least have a minimum impact... that would likely only go up when demand increases.
But like I outlined above... the gondola system has several huge advantages up front when it comes to efficiencies of movement. And the one big take away I had from my nuclear engineering class was.... you can radically simplify your analysis when you simply throw out the portions of the equation that never have a chance of being materially significant. Now in that context, we were talking about how things that were huge orders of magnitude different... there was no reason to calculate the little guy... but the same philosophy applies here. When something has huge multipliers as a head start... it is going to take a huge disparity for the other side to catch up.
And we already know that ICE is a very inefficient form of combustion and diesel is not a really clean burning fuel to start with.... compared to bulk generated electricity that goes through very efficient motors. Then start to think about all the energy, time, labor, etc spent maintaining many vehicles verse a consolidated system, etc. it’s a tall hill for the buses to climb..
As for not reducing bus counts... Disney isn’t going to run more buses than it needs on those other existing routes. So excess supply will simply be cut. And for weather closures, Disney already has a system for elastic demand... maybe they simply add a tad more capacity to that pool (which would not be the same as a full time route)... or they simply accept that wait times will exceed the norm in those situations.
Plus, remember that weather is something that can be forecasted... even if not perfect.