The statement that the real aircraft does not have a certain item is somehow relative. OK, the aircraft did not have an ALT HOLD mode when it was released, and I get that you are modeling the plane with that state in mind, but many of the Arrow III aircrafts that are currently flying have seen they're avionics updated, some having completely new systems with modern autopilot units.
I get the vintage look and feel of the plane and all that staff, and I fully endorse you to keep it as true to real live as you are doing, but, at least from my perspective, it would be great to have the option to fly a version of the Arrow with a little more modern avionics, like a more complex autopilot and maybe some more GPS options (like a GTN for example)... I don't know about you guys but for me that would be really fantastic, even if you happen to launch it as an addon...