If the terrain is suitable for walking, archies have two choices. They can design for carts or design for walking. Because people choose to ride on courses which are very walkable, does that then mean archies should design new courses to ride? I don't follow your logic, if that is your logic.
One of the best ways to promote golf and thus recruit golfers, is to emphasize the health aspects of walking. That can only be done if courses are a reasonable walk.
First of all, there are very few new courses being built in the U.S. and Europe. I have zero doubt that closings will continue to far outpace new construction for many years (wait until the Boomers can no longer play for physical or economic reasons).
I was primarily addressing existing courses in the U.S. , many which have or will be undergoing considerable updating, and in the context of providing multiple tees to satisfy the needs of a larger range of golfers. There are some existing courses with enough elasticity, but I am not advocating adding sets of formalized back tees (though an occasional rudimentary "monster" tee for a special competition might be ok).
The choice of the architect in some cases is not binary. Depending on the purpose of the course (say to sell residential lots), site characteristics, and regulatory requirements, it may not be possible to build an easily walkable course. But I can think of few instances in my part of the country where it would make sense not to include carts in the design.
Whether we like it or not, at least in my region, if carts are not available, golfers will not play. Even requiring carts to stay on the paths dampens demand.
If I was an owner or an investor, I would want my course to be desirable by all types of golfers. I would expect my architect to design a course which serves the most, including providing for the use of carts on an easily walkable routing (which would always be my choice if golf was the sole purpose for the course). I don't see where the logic is missing here.
As to the health aspects, I offer my son as an example of someone who is reasonably smart, very fit, and fairly representative of his Millennial group. He typically rides a cart on easily walkable courses and works out three or four times each week. We can extol the virtues of walking 'til the cows come home, but like with politics and so many other things, conversions are likely to be few. Ultra-high pricing, say $40+ per half cart, is about the only impediment I can think of that might bend the curve. It is not by accident that caddie programs are dead pretty much anywhere they are not mandatory since the advent of the $80+/bag middle-age professional caddie.