Yes, the 722K had the plug in module for OTA. The 722 had a built in OTA tuner. I had a 722 before my first Hopper and it had an OTA coax input. By the time the 722K came out the business decision was to make the OTA tuner a customer elected decision. If you wanted OTA you had to purchase...
The Hopper Duo can serve 2 users but that second viewer needs a Joey. That Joey, incidentally, provides an HD picture which the 722 didn’t. Duo means 2 users over 2 tuners, nothing more.
What you are suggesting would require an internal OTA tuner. That would mean every Hopper would have an OTA tuner. The majority of subscribers do not have OTA antennas so that internal tuner is a waste of equipment for most. That translates to expense for little need.
Dish never had a Hopper 2. What we often term Hopper 2 is actually Hopper with Sling. There are 4 versions of Hopper; Hopper 2000, Hopper with Sling, Hopper 3, and Hopper Duo.