Because if Network hash rate doesn't at least keep up with computing power, it can be outcompeted, and lose the 51% battle?
You seem to have forgotten the second half of Moore's law. Hardware also gets cheaper as it gets more powerful. Fees will more than keep miners in the latest hardware.
I think you are one step closer to realizing the problem.
Hash must always grow.
Nope, it must not. In order for the network to be secure, 51% of hashing power must remain in "white-hat" hands.
Even if a 51% attack happened, it wouldn't do much, nor last long (as the other miners could simply buy cheap hardware and knock him back under 51%).
It's more profitable to play by the rules, than to try and subvert them.
See:
https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power