Unless.... the trading price drops to "mining unprofitable" value until the Pool operators themselves came up with an idea to earn "extra profits" by doctoring the next blocks.
As discussed in other threads, achieving more than 51% of the total hashrate isn't too far-fetched if two or more of the current mining pools merge together.
So if bitcoin's value became obsolete, this kind of possibility may happen...
Not saying that mining operators will revolt against Bitcoin but based on the statement: "How strong is Bitcoin against 51% attack".

The whole idea can't be based on the mining equipment/operation cost at all, but the whole setup including the price.
With the current setup, we are not that safe if a resourceful adversary plans it well, given that the Bitcoin trading price drops to an alarming value.

We have been there before and when a huge Pool reached a percentage close to 51%, a lot of Pool members shifted their hashing to another Pool. Why would they willingly do that? Well the answer is simple.. If a double spend were done, investors in Bitcoin will question the security of this technology and the validity of the transactions.

A massive dump will follow and the price will take a nose dive. An attack of this magnitude will cause panic and the attackers will gain very little in the long run, because the price will keep going down. The cost to sustain such an attack, also outweigh the gains of doing this.