I agree with this to an extent -- I don't think there is a comparable theme park experience elsewhere (at least for what specifically interests me about theme parks). Universal is worth visiting, but I can't spend more than 2 days there. That's why I will probably continue to visit Disney once every few years.
With that said, I live close enough to WDW that I can drive there in a reasonable amount of time (about 6.5 hours), which makes things much easier. Don't have to spend money on plane tickets and have full control over my travel timing. I'd probably go less or maybe not at all if it was going to require air travel. There are far too many other places I want to see/return to. If all things were equal (cost, time, etc.) between visiting WDW and going to London, Paris, Rome, or so many other places, I would choose the latter every time. And if prices keep going up while service, food quality, maintenance, etc. stay the same or decline further, WDW will lose even the occasional trip. I already have no current plans to visit.