It's not one pitch 70% of the time, but back to Randy Johnson, he threw 52.8% FB, 38.6% Slider, so basically a two-pitch pitcher. He threw the occasional change-up for most of his career, then gave up on that when he started throwing a splitter more at the end of his career. Ben Sheets threw 63.1% FB / 30.7% curve ball for his career. If you're going to make it on two pitches, they'd better be darned good pitches. Hader could potentially do that, but would obviously be much better off if he can get the change-up to league average. Lance Lynn has thrown no lower than 70% fastball in his career (in 2016, he threw 85%). This is the main reason why I wouldn't come close to signing him.