Apple ALWAYS does this! Look at history.
The first round of Intel machines used the same shell as their predecessors.
The A7 came in the the same looking iPhone 5S shell.
Changing a device's case is way more expensive than you think, because the entire tooling and assembly flow has to be changed. The first round of new SoC is in the old cases for "safety" reasons; get all the basics out the way and working. Then when that's done, figure out the new things that the new CPU/SoC allows you to do, and redesign the cases appropriately (after the A7, for example, Apple moved on to the much larger screen iPhones).
Consider eg the MBA camera. Yes it sucks. But why bother just putting in a slightly better camera? If you're going to do it right, you use the iDevice camera and provide FaceID. But now you need space for the IR camera and receiver, so you have to change the bezel. And so it goes.
The present THIS year is faster CPU. If you want a new case, that'll be the present probably next year. (That would be my guess, though Apple may have their hands so full with moving the entire Mac line over that it may have to wait two years.)