That's a big problem then. That means once the block mining reward turns to zero most miners will have to cease their operation to keep themselves in the black. If that happens, the cost of a 50% attack significantly decreases.
And we're not talking black or white here.
It's not that miners suddenly will earn zero money.
It will just decrease to the point where it may make sense to sell some of the equipment. That equipment might still be somewhat profitable, just not as insanely high as it is now.