[...] It does matter if the "dollar" value of BTC has doubled since then. Since if hypothetically the value of Bitcoin doubles after the halving, then there is still the same amount being paid for security
But since the value of BTC has doubled, the thing that you're securing has doubled in value, so you need twice the security. If a $5 lock is just-good-enough to protect a $100 bike, is it also good enough to protect a $200 bike?