Bitcoin is probabilistic. The scenario you mentioned does not make much sense, because nobody would waste his resources in that way. There are other ways to break consensus, namely through Alt-Coins. If you have developed a far superior mechanism people would flock to that.
I agree that this scenario is highly unlikely. But I am more concerned by decentralization credibility issues.
Checkpoints set by small group of people - hurt faith in Bitcoin decentralization, which is the main advantage over other currencies.
On the other side - checkpoints solve very little and rare problems, which do not worth losing decentralization credibility. Moreover such problems can be solved in other ways.