Your belief that Bitcoin has to be constrained by the belief that hardware will never improve is extremist, but regardless, your concerns are easy to assuage: there is no requirement that the block chain be stored on hard disks. As you note yourself the block chain is used for building/auditing the ledger. Random access to it is not required, if all you care about is running a full node.

Luckily this makes it a great fit for tape backup. Technology that can store 185 terabytes per cartridge has already been developed:

As you could certainly share costs of a block chain archive with other people, the cost would not be a major concern even today. And it's virtually guaranteed that humanity will not hit a storage technology wall in 2015.

If your computer is compromised then all bets are off. Validating the chain on a compromised host is meaningless.