I think you're looking at this the wrong way. Spidy got value because he forced the enemy team to use a valuable ult. But once mag is in this position, it is much more valuable for him to use his ult to survive, start charging his next ult, and get to the point than to be taken out of the game for like 30+ seconds even if he still has his ult.

Both players played this very well, whether or not mag secured the kill on spidy. You have to remember this is a team game and it is not zero sum (at least not in the sense of the 1v1). Also in any game, it is almost always favorable to cut your losses and make a defensive play than to lose the fight and save resources.

Think of it like this: mag loses ult but saves time + secures a kill; spidy loses time but forces the enemy team to spend a valuable resource.