I would love to see the Pencil become a common accessory that works across Apple's stable of devices.
I think it's inevitable that in a few years time Apple will release a large format touch based desktop/tablet hybrid with an iMac-like form factor. It would be awesome to have Pencil on such a display.
I don't get OPs criticism - I don't think anyone *wants* Pencil limited to iPad Pro, but we simply expect that is Apple's plan for the near future.
Knowing Apple though, I feel this could go either way. I think either the Pencil becomes a key Apple accessory, or it's going to slowly fade away. I don't see it being an accessory that is well supported and updated but exclusive to one product. They either expand its compatibility, or the current Pencil is the only one we'll ever see. That's my prediction.