Some random thoughts (and trying to keep this short, so very rough) ...
once said, "It’s Difficult to Make Predictions, Especially About the Future."
Whether an event or attraction is secular or religious, organic or artificial, new or ancient if it can be exploited to make money it will be. Or, perhaps more accurately, it already has been and is being currently exploited to make money. If an event or attraction needs to be created for that purpose in the future it will be if there is enough money in it.
Just as the Americas did not exist before Christopher Columbus discovered them, San Miguel de Allende did not exist before Stirling Dickinson discovered it. And it has been discovered again by readers of Conde Nast. and, Travel and Leisure, the NYT, and ...
SMA has passed some threshold and has a critical mass of attention that will continue to fuel its growth as a tourist destination until and unless something major changes. It matters little in the big picture, I think, whether the tourists are coming from Mexico City, NOB, Europe, Asia or elsewhere.
Except there is a difference between tourists driving here or flying in to a nearby airport in terms of local vehicle traffic. And, the local government is currently working to increase the amount of local traffic (in some areas at least) with current and planned road, glorieta and traffic signal projects. (If you reduce traffic congestion and improve traffic flow the expected result is an increased amount of traffic.)
On the other hand, cartel murders in SMA may impact the tourism business and seems to already have had an effect, especially for those outside of Mexico. Although tourists and retirees are not generally the target of this cartel action it is bad for the tourism image. As I see it, there is little the local government can do about cartel-on-cartel violence. Things can be done, just not at the level of local government.
Against that is the backdrop of environmental and climate change. I expect climate change to get much worse before it gets better. And we have declining water tables with more and more arsenic and fluoride contamination.
But I expect (hope) that the SMA climate remains great for the rest of my lifetime. Maybe we will have to truck in water (from where?) and, use rainwater collection where ever possible.
Relations with the US are a factor and hard to predict. If the US does not welcome Mexican tourists then that benefits SMA tourism business from Mexico. If relations sour, Mexico could retaliate and make it less desirable for US immigrants to live here.
The drug business and cartel violence is another big factor and hard to predict.
If those two factors stay about the same, then my predictions for the most likely scenario is more of the same:
SMA will continue to be a retirement destination for immigrants from NOB with enough money.
SMA will continue to be a tourist destination for those from around the world with enough money.
SMA will continue to be developed in a manner to attract tourists and retirees.
The biggest winners in this will be the moneyed interests driving these investments.
The biggest losers will be the regular local Mexicans in SMA that are getting squeezed economically.