The problem with your theoretical attack is, as per the Bitcoin protocol, the target (which determines the difficulty) cannot change by more than a factor of 4 between rounds.[0]
You can't rapidly jump between difficulties like that.
Even if you controlled 50% of the mining pool then it wouldn't increase by more than a factor of 4. (It would jump from 2/3 to 1).
It's about the fact that you mine more efficiently not suddenly mining a lot more bitcoin. (You actually mine fewer blocks overall but at a larger reduction in running time costs than the reduction in mining.)
You can't rapidly jump between difficulties like that.
[0] https://en.bitcoin.it/wiki/Target