Hrm? Elaborate a bit on the practicalities of your cheap attack where one "must only make the reward higher" will you?
Occurs to me that a fair number of people own many 2010-2011-ish days worth of mining production. High GINI here. Or a guy (cough, cough) could steal (aka 'confiscate' if you are in law enforcement) a chunk from such a person and obtain some pretty heavy firepower especially since it is multiplied by that by 4 at the next halving (and we all know that transaction fees won't be dick.) Firing up a pool for whatever disenchanted and powered-down rigs who might be interested and one could reward them relatively generously. Enough to get 51% just on economics (not to mention the other hassles)? I could see corner cases (such as everyone hating Bitcoin for some reason) but honestly probably not (so you win.)
That said, even just hosing things up for a while when everyone is counting on Bitcoin for their daily dose of smack and you could create some frightened and stressed out sheep like the guy I mentioned yesterday. If an attack takes this form the miners could be mining for real (and collection normal fees which you could
modestly subsidize) but in such a pattern designed to produce unhappy resonances and other nuisances.
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.
Luke-jr raped some altcoin once, that's a super cool story but that doesn't really relate to Bitcoin in any way other than being some lame anecdotal evidence for your nebulous argument.
I wasn't following it closely (and was a little disgusted) but as I recall, all lukedashjr did was mine for a while then quite and it had a devastating impact on whatever coin it was. Hell, the same thing could happen to Bitcoin
without any attack if the price drops steeply or suddenly (but that could never happen, right?) and/or a few large clusters were molested. I'll call this an '
Invisible hand of Adam Smith lukedashjr Attack.' Maybe the adjustment algorithms have been patched up to preclude this, but I doubt it...such things occurring proactively seem rare in Bitcoinland.