Either way to me, it's a risky gamble to try and thread the needle on a 31 year old wide receiver who's missed 15 games in the last two-season, hoping for what would probably be his final year of WR1 production. That's just a very narrow window to fit the football into. I don't think his talent has deteriorated, but you know how it is, I could do things at Age 28 that suddenly became more difficult (seemingly overnight) at Age 31.
I don't know you, and I don't want to come off as a jerk. As a fantasy owner, I care more about a player's relative value. You discussed that this could potentially be Kupp's last season as a WR1. His ADP is WR29. By definition, those are the types of players you should be targeting, not labeling will-not-draft. All the risk and associated issues you have described about Kupp are baked into his cost / draft stock / discounted acquisition price. Similarly, if Kupp plays consistently, IMO, there isn't a good path for Puka to earn back his WR8 draft position. So on my draft board, Puka is the one with the will-not-draft label. I think he will have a negative ROI (as his increased scoring was temporary and situational). If you don't get a good vibe, then don't draft him. I would likely counter that other WRs available at the same point in the draft won't have the value or the upside, but there's no shortage of players to draft and plenty of other options available.
We could debate whether any player is more of an injury risk than any other . . . all guys are at risk of getting hurt on any play, and it's the severity of the injury that is pretty random. One guy gets a bone bruise and misses a game, another guy gets hit in the same spot and tears a meniscus. One guy gets a minor hamstring pull, another guy blows out his Achilles. As Bill Belichick used to say, tell me beforehand the play when a guy is going to get injured, and I will sit him out on that play.
I've been playing fantasy for decades and researched injuries quite a bit (for this very website). Yes, older players are more prone to get minor injuries, but I did not see any conclusive evidence that older players suffered a higher rate of major, season-ending injuries. Also, some players have had serious injuries that largely didn't matter for fantasy, as they happened at the end of a season instead of the start of the season. Bottom line, every player is an injury risk, and there's not a whole lot you can do about it. The one time I ever drafted Tom Brady, he tore his ACL in Week 1. Injuries happen, and all you can do is deal with them.