You're making two mistakes that I can see, both related to the idea of "shrinking" the set of front or back wheels into a single wheel.
Rather than thinking of Ackermann steering as (conceptually) a single wheel, imagine expanding the single front wheel of a tricycle into 2 wheels. At first, the tire gets wider, then splits into two tires, then they get further apart — but the axles of the two wheels remain on the same line. In other words, you end up with a "steerable front beam axle" like you'd find on a toy wagon — not an Ackermann system:
You could think of an Ackermann system as two bicycles welded together side by side, noting that connecting the front wheels is not solved by simply forcing their steering angles to be equal. Instead, you might look at techniques like Burmester's theory to design the proper kinematic linkage between them. (In the Ackermann solution, it's a 4-bar linkage.)
For the rear wheels, you're ignoring the ability to lean. In other words, a bicycle is not simply a tricycle with zero spacing between its back wheels; leaning is an integral part of maintaining stability with only two contact points.
(shifting weight to remain stable, via "Tricycle Steering"), (leaning to turn, via The stability of the bicycle)
Leaning is more of a dynamic discussion than a kinematic one, but worth noting since it affects the tires — bicycle/motorcycle tires have a rounded cross section to accomodate that lean, while car/motor-trike tires have a flatter cross section.