This has happened in the past, see http://arstechnica.com/security/2014/06/after-reaching-51-ne... .

I agree the required power must have grown since 2014. But not knowing by which factor, one can only assume it's still feasible.

I can't find any source for it, but I remember reading not so long ago that 2 or 3 mining pools were responsible for 2/3 of the total bitcoin mining power. That's not something in favor of trusting the blockain as infallible.