A player’s value is their value over replacement, not necesarily their absolute scoring production for a given year.
For example, would you rather have a guy who scored 20 points per game but you knew would miss 8 games per year, or a guy who scored 12 per game but was reliable for 16 games per year? He’s scoring 192 points per year vs the other guy’s 160.
But not so fast. You’re not leaving that position empty while your guy is injured, scoring zero. You’re going to get a replacement-level player. What if the available replacement-level players average a score of 10 points per game? Then you’re getting 820=160 + 810=80 = 240 points vs the 192 points.
You’re looking at Gronk and saying “he’s the 12th scoring TE, that’s worthless”, not “he’s a guy who scored 19.5 points per game, by far the best at his position, during the games he was healthy”
No one else in the league is even remotely close to beating the next guy up in per-game production. Gronk has about a 50-70% edge on the next guy up at the TE position, compared to say WR or RB where the edge that first has over second is like 3%.
He’s also not really injury prone. It’s not like the guy is a weakling. Because he’s so physically dominant, the refs let the other teams get away with a lot of stuff they’d never allow on most players. In that way he’s a lot like Cam Newton. The fact that he gets injured more than Newton isn’t being injury prone on Gronk’s part, since I think most players would have a similar injury history or worse if they had to take the beating Gronk takes, but more a sign that Newton is superman. Gronk’s injuries have been all sorts of things - this year it’s a punctured lung - rarely the same thing twice. He played 15 games last season and another 15 the season before that.
In any case, Gronk was healthy. He was killing it. He was propelling me to victories. I have no idea how you can discount that because hey, he was injured before, so I didn’t really lose anything. It’s still a big loss for me.
As far as Kelce for Gronk - anyone else in this league would fist pump slam dunk that deal in 2 seconds, so I think you’re fundamentally misunderstanding a lot about fantasy football strategy.