Yes. It means that to attack Bitcoin one must only make the reward higher to do so. And if the reward for supporting Bitcoin tends toward zero the cost of doing so is fairly minimal.
It's the margin that tends to zero, not the reward.
Your attack is self-correcting, manage to make the global hashrate drop, everyone's rig is suddenly more valuable.
Tell us all a bit more about the magic of hashrate adjustments and the lightning responses. /sarc IIRC, some sha256 alt coins which use the same mechanism found it more practical to just re-distribute new binaries than to wait it out when faced with PoW attacks.
Yesterday there was an hour when only one block was mined in an hour and a fee-paying transaction didn't get into it. One quite experienced user started to freak out. This is yet another in a long list of reasons to stop dreaming about using native Bitcoin for the masses to buy trinkets. If sidechains were what was being used by the majority for their daily energy-drink needs it provides a rather nice buffer. The sidecoin could not inflate or deflate while an attack on native Bitcoin was dealt with, but most user's would probably barely know it.