I know health risk is always there, but I don't blame him for turning it down if he wants to bet on himself. On the open market he'd easily get over $40m AAV two years from now. He's making $17m this season. Two more years of arb he's likely looking at, what, $50m (give or take) over the next two years? So if someone will give him over $400m two years from now, plus the ~$50m he will get in arb over the next two years, then why would he sign that Nats deal? Only reason would be if he was terrified of injury or performance over the next 2 years, and he clearly isn't.
I agree it's tough to turn that much guaranteed money down, but if he feels he can get more, possibly much more, then good for him to bet on himself.