I don't tend to get upset with players for things they can't control. DMC was never going to be good against the P&R or defending the rim just because of his lack of lateral quickness and vertical explosion. You take the good with the bad. Considering his strength at 18 I don't think defending the post will be an issue for Sengun. But if his shot doesn't develop that's a major issue. And if he can't be an efficient post scorer against consistently longer defenders that's also a major issue. No doubt about that.
But that's different than what bothers me about Bagley. It's not physical limitations for him. It's a lack of development. He's still very left hand dominant, he still doesn't set good screens, he still misses defensive rotations, and his scoring is still largely from transition opportunities, offensive boards, and post ups. If he played good defense he'd be a good modern NBA center, but he doesn't.
I mean, if the Kings don't find a deal for him (and especially if Holmes leaves) I'd roll Bagley out at the five and let him sink or swim. Because at this point a traditional PF on offense that is bad on defense is one of the more useless player archetypes available. I think other than becoming a backup elsewhere Bagley's only real path to success in the NBA is becoming a modern NBA center.
I like Wagner a lot so I'm not going to argue against taking him, but he's also not a sure fire success on the NBA level. If he's unable to keep pace with smaller wings and not strong enough to handle bigger forwards he's also the bad type of a tweener. And his shot needs more work than most people seem to acknowledge.
There's a reason why half the players in the lottery bust. It's largely a crapshoot and even "safe" picks aren't always immune from that.