He's another year older with serious injury concerns. Step away from your projections for a second and look at the person. He cannot stay healthy and cannot consistently throw strikes. In every sport there are always enigmas who never realize their potential. Santos clearly is one of these guys.
Carlos Marmol in 2010 is the perfect example. Fantastic xFIP but clearly had massive control concerns. The older he got, the quicker all of those concerns caught up to him and he went full gascan (likely more wear and tear on the arm resulting in velocity dropping, etc) . I have no doubt that you would've been saying the exact same things back then about Marmol that you are saying about Santos now (actually, you should've been more confident in projecting Marmol as he had a much larger track record).
Projections are not fool proof. Life happens. Santos clearly shows signs of breaking down. If he's still in baseball in 3 years I'd be shocked.