It doesn't make sense at first glance, but then historically Apple has used long transitions to move from old to new Macs.
The non-retina to retina MacBook Pro transition is a prime example. They moved the latter to the higher end option whilst keeping the former as a entry to mid tier, and over time reduced the dependancy on the non-retina models by slimming down the configs.
It always occurred to me that both the current 13 and 16 inch MacBook Pros required new chassis, even the 13 inch though being virtually identically, would have required new tooling and manufacturing just for that Magic Keyboard - the only notable upgrade. Suddenly dropping this design after such little time wouldn't make any sense for Apple.
So my guess is that the current chassis for the 13 inch will be here to stay for at least another year. Apple will replace the Intel CPUs with ARM, and offer configs of a similar nature to today; Good, Better, Best. The rest of the industrial design therefore need not change.
The next gen. 14 inch will be introduced later this year as a highest end option, and gradually, the 13 inch will become a lone product that is the entry level model.
Same with MacBook Air. It surely can be no coincidence the thermal design of the current device is completely unsuited to the featured Intel CPU.