If you want one bike to do it all, road, gravel, combination of both then I can see the advantage of being able to go from 25 or 28 to 32 or even 40 tires as needed but I was referring to road bikes and most caliper brakes will work with up to 28's...and I cant see any advantage in anything wider than that for the road.
Not sure a road race or even an "endurance" race bike is really the best choice for gravel riding and I think that as much as the one bike concept is attractive, I don't buy it. If you willing to spend big $ on a bike, decide what you want it to be.