As I noted, road diets don't work everywhere. One size definitely does not fit all. I've seen them work well in the urban one-way street example. The 4-lane-to-2-lane with turn lanes works for a short distance in a business area where there's a lot of turning activity and bicycles and traffic is slow already, but not so well where all 4 lanes are actually used by flowing traffic most of the time. And if the 4-lane already has a center turn lane or frequent turn pockets then a road diet is just asking for trouble.

There's a point where simply widening the road (or adding a track after maxing out siding tweaks and CTC and PTC) is needed. Or improving a parallel route so congestion can be reduced (you don't want to eliminate it - congestion in cities is an indication that people actually want to be there) by moving through traffic over there. The one-way-street example, for instance, is in a gridded street pattern where there are multiple parallel alternative streets within a short distance on all sides. So traffic can (and does) spread around.

The dumbest thing I've seen in pursuit of "traffic calming" and "road diets" is turning modest one-way streets (2 or 3 lanes) into two-way (one lane each way) when the one-way street carried considerable traffic - that can cause local gridlock if done blindly. "If you don't build it (or take it away) they will just switch to bikes and transit" doesn't always work, or else "they" just go away leaving your local businesses (and eventually your tax base) with a problem.