Is "dumb down" a euphemism for "make easier?
If so, then of course carts dumb down a course. Many clubhouses are on the top of hills, and so many courses finish with uphill holes. Riding takes away the element of fitness that these uphill holes test. And of course this isn't true only of finishing holes. If you've climbed up to any green location, you're putting with your heart beating just a little faster. Isn't that part of the challenge of golf? Certainly not the central challenge, but it's a part of the game.
Understand, I'm overweight, and walking 18 holes is a hell of a challenge for me. You can call me names if you want, stupid or lazy or fat or whatever. Walking a course is what I need to be doing, but sometimes I ride. I understand what I'm losing by it, and what I'm missing out on, but playing is better than not playing. I'd want to train some before going up to Bandon, or taking on Ballyneal or any other course that requires that I walk (assuming that they'd let my fat butt on the property). But even without all of the aesthetic or historical or moral or "I'm better than you" arguments regarding the use of a cart, it can't be denied that it removes one challenge from the game in that it makes it physically less taxing. Not much of an issue for the truly fit, but for others, certainly a consideration.