Why is the payment channel idea not good enough for most transactions that do not require censorship resistance?
There's nothing wrong with it
if that is what the free market decides.
If forcing transactions into payment channels requires a production quota on block space, then I think there
is something wrong with it.

Moreover, I don't think it would even be possible to enforce the quota, since the pressure indicated by the brown shaded region in the lower chart would provide impetus to fork the protocol (larger block sizes) to satisfy the demand unmet due to the quota.
Source:
http://bitco.in/forum/threads/gold-collapsing-bitcoin-up.16/page-12#post-392They can't fork away into a more centralized coin if they wish to. At some point we should expect Bitcoin holder to recognize what makes it different than other solutions (hint: not transactions).
You keep posting these charts as if the negative externality was not obvious....