I agree in principles, but there's a difference. A system able to break a strong encryption could cost more than the world GDP and is not reallistically feasible, while owning 51% of bitcoin power is possible - not cheap, but possible.
It is the same with blockchains. It is just that there isn't one that exists (yet) which has enough distributed hash power where it's infeasible for any actor/group to ever get 51%. Bitcoin is just the best example so far, some might argue it's still in its early stages, or that another blockchains will surpass it.