Yeah, of course Apple has done this forever. Still, a faster processor is in no way comparable to actual FEATURES. FaceID, Pencil2, "bezelless", ....you name it.
And that design and features already exist in smaller form factors: iPhone XS and Max... No, it's not rocket science. They simply don't want to. Even coding wouldn't need to be that difficult, they already have to code for various screen sizes...
I can tell you why Mini sales were low. Because it essentially was running on a years old design and features. Steve Jobs said noone wanted a small iPad and that you needed a certain screen size to appreciate all the features. Yet, eventually they introduced the Mini and it sold like hot potatoes at the time.