It’s more the nature of the T2 and the Secure Enclave than any tribal thing they may want I think. The T2 is a separate processor and it runs off the Secure Enclave which is separate memory physically not accessible from the main CPU or RAM. It’s also. Used on iPhones and iPads, and is where Apple stashes sensitive encrypted info like biometrics and Apple Pay information. And both require a very specific custom motherboard with the parts all matched to the T2, hence the restriction.
I think the part where they aim to be “cool” is where this comes from more, as really no other laptop has implemented this sort of hardware based security and one could argue it is over the top.
It isn't just the chip hurting Apple. It's the entire persona of Apple, it's business ethic, it's image and especially the price to value ratio. A notchless, thin bezel smartphone with dual cameras on the back and a low power front facing camera is not a new iPhone, it's an older Android phone at less than 1/4 the price of the latest iPhone X. It is seen as dumb to buy Apple now. Not a single Apple product can claim to be the best in class. Not one, including build quality. What Apple has left and is worth a lot, is cachet. As soon as that becomes dull, Apple is done unless they move into a new market. The numbers don't lie: iPhone is becoming passe. The Macbook Pro is a beautiful laptop, but at $3000, you can do better in power and equal in design with a Microsoft Surface or a myriad of high end PC laptops. All of which will gladly accept Mini DVI, USB-C and standard USB, and will also have a standard mic and headphone jack. And you will be able to take them to a myriad of places to have them serviced when they need it, as well as be able to do a lot of the work yourself if you are so inclined.
Apple sucks every which way you look at it. The Loyalists keep it alive. BTW, Apple fell behind Walmart and Amazon for the top 3 most valuable companies. Amazon is 1st, then Walmart, then Apple.