Many of the same points that have been made before about how completely unprepared QB's from the Air Raid system come into the NFL. This year teams will make mistakes like this with Mahomes and Webb.
So, they're right to a point in saying Goff can't play in the NFL. Last year he clearly couldn't. But, will he learn and then be able to apply the skill set he has to that learning so that he can become a legitimate pro style franchise QB? We don't know the answer to that yet, but I do think it's fair to say the odds are not great.
Can Mannion be the guy? I doubt that. He's got the opposite problem. He was groomed in a pro style offense in college and "should" have all those advantages to go along with very good physical attributes. Problem there is, he's simply never performed as well as you'd hope or expect along the way. I so wanted to love Mannion when he was at Oregon State. Gave him a lot of rope and by the end of his last year there, he convinced me he wasn't going to be the answer either. It's always possible that he's figured something out since he entered the NFL too, but I wouldn't bet on that.