It really surprises me that he didn't get an extra year or two tagged on to avoid opting out

The cards gave him an extra year at the end and added the 22-23 opt out at the time of the deal so maybe both sides agreed there would be no Furthur adjustment

#2 makes a fair point but ... still seems a bit odd to me. That age 36 add-on is a mere $15 M which is already pretty odd -- and happens to equal the amount Colorado is paying for ages 33-35 which I suspect is not a coincidence. He could still be a quite productive player at 36 -- I would have thought maybe something like a team option at $30 and a player option at $15. Or at least get something like that in exchange for not opting out. It's alsmot like Arenado is planning on not being very good at 36 -- which might be rational but is rare among players.

He might actually beat 5/$144 if he went FA but not by enough to take the risk. To be clear, he might very well not -- he was awesome this year but 2020-21 and his age might be scary enough that nobody will want to give him something like Rendon's contract through 36. (I keep forgetting about that one -- Rendon is one of the highest-paid position players.)

So 5/$144 for Arenado 32-36 is "fair" but 4/$130 with 1/$15 tacked on is weird. I guess he was pretty happy to be out of Colorado.

The cards gave him an extra year at the end and added the 22-23 opt out at the time of the deal so maybe both sides agreed there would be no Furthur adjustment

Seems more likely that the pitch would be 'we'll tack on 1/15, which we know may not turn out to be much, so we'll also give you some opt outs.'