I fully agree with the fact that I shouldn't have to pay a subscription for supercruise. That said, I also included it on my must have features. Its the future and I think it will help long term resale and I like having the additional braking safety features that come with it.
I fully expect that in the next 3 years when it expires, the general public will have rebelled against the automakers and they will allow the service free. Or as was already mentioned, big brother may force their hand. They may come out with better features that are only available with subscription and what we have today is free.
Worst case, I'm wrong and have to pay to play. I see it as something I want primarily for road trips and would likely renew for a month as needed and cancel. I do the same with Onstar and XM radio. Nice to have on long trips, but not worth paying for it year round. Again, I think this is stupid, but was a risk I'm willing to take.