I have had the MBUX for 3 months now. You are partially correct. MBUX comes with full functionality except for the hotspot. You don't need a subscription for the traffic, directions or software updates. It all comes with MBUX. (for 3 years...who knows the cost after that?). The AT&T contract is only to enable the wi-fi hotspot. It is pretty cheap. I think I pay $5 a month for it, unlimited. AT&T wanted $10 a month to add it to my existing account, so I just set up a seperate account. When you get yours, you go into settings > Hotspot and it gives you the screens to sign up. It made me the special offer of $5 a month, add to existing account and one other. I selected the cheap one. Works great and 10 devices can connect at once.
The cool thing is, to connect, you go to that wi-fi hotspot settings screen and it will display a QR code. On the device you want to connect to the hotspot, open the camera and aim at the QR Code. A dialog box pops up on your device asking if you want to connect. Say YES and it connects. For future connections, it just connects automatically when the van hotspot is available.