I suspect at some point China will forcibly demonstrate this fact. After all Tor is already banned there and VPNs are regularly disrupted. So, ban port 8333, block websites of exchanges and web wallets, forbid banks from wiring to the primary exchanges, announce that anyone who advertises acceptance of non-licensed currencies will be jailed. Done. All of those things are easily within the abilities of the Chinese government and some of them are quite easily done by other governments too (there's already a large financial sanctions infrastructure in place in the west).
You can't have a currency which people are afraid to advertise acceptance of because the utility of a currency is the square of its participants. An outlaw currency isn't even useful to outlaws.
If you want to build a currency that can operate in a world where accepting it leads to immediate punishment then go right ahead and try, but it won't be easy and block sizes are the very least of the problems you'll have to solve.
I don't think that it would be technically unfeasible to devise such a solution, nor do I believe that it is as trivial as you make it sound for a society to stamp out things that the leadership does not like. If so, there would not be such a thing as 'contraband', and it is absurd to turn a blind eye to the existence of such things or to their value (which happens to often rise dramatically due to artificially straining the supply/demand curve.)
I don't know how prevalent CP is and I imagine that the problem is blown out of proportion for political reasons, but I am sure that it does exist. This in spite of nearly universal negativity on the part of almost every society and strong support for enforcement it has not been stamped out.
In contrast to CP, efforts to interfere with a distributed crypto-currency would probably be seen for what they are by large segments of the population; self-serving and unfair policies designed under a crony regime to benefit their well connected friends. In short they are likely to receive much less public support and subject a much broader segment of the population to abuse. This will impose some limit on the level of punishment that a state can impose on those caught participating in the economy. Of course that will vary from state to state, but we are talking about a global phenomenon here.
I am speaking about a 'reserve' currency role. Arguments about the ability of the state to interfere with Skittle purchases are in a different category. Those are not the kinds of transactions inherent in a 'reserve' role, but I do not believe that value in a 'reserve' capacity is negated because an item has no realistic 'exchange' function. Certainly it is not what we see in precious metals.
I do suspect that you are right that Bitcoin will simply not be the solution which ends up being a trusted reserve currency. This is not because it is could not be. Indeed, one has to work at it to make it NOT fall into this role. It is more that much of the development team simply sees no value or role for such a solution and if anything seems to consider it an undesirable thing and something to be actively avoided.