I understand your point of view. If the rest of the mods agree, we will renegotiate.
Just a couple of points to consider:
There is value in the prospect extension the Yankees are able to use on Pasquantino. They have 5.5 yrs of control over him vs 1.5 of Castillo. On the open market, Pasquantino should land a $10 - $12m AAV (using Nootbar as best comp for age and performance 4 yrs @$12m AAV). There's big long-term savings for the Yankees...if extended with current rankings, about $30 - 40m over the life of the extension, which alone is well worth the $16m to pay down Castillo's salary.
The injury risk on pitching is significant in my evaluation here. For context, I have 7 SP's on the IL right now (6 of those are due to TJS)...so I have a little PTSD when it comes to pitching and forces me to consider the range of outcomes.
My worst case scenario is Castillo blows out his elbow soon after I acquire him, taking him out of action thru this year and next. He's not back until he's a FA and I've given 5.5 yrs of Pasquantino away for nothing. Best case scenario is Castillo is the best SP in baseball, in which Pasquantino still likely outscores him this year and next and the Yankees still have him for 4 more years after Castillo becomes a FA.