Couple things here. I'm in no way speaking in absolutes; just opinion. And this is not an attack on you or anyone for the record.
1. "It's not about changing opionions, it's about performing." --
But that's just the thing, performance develops AND changes opinions of a player. One may say 'AP has lost a step'. That changed the opinion of said person due to his performance dip. Why? 'Cause of his age'. Well, that's probably right, but the opinion of AP was different a few years ago.
2. "I don't understand why questioning a player, regardless of who, is deemed as a negative." --
Questioning a player is part of observation. Take for example our coaching staff: They observed collegiate players before the draft thoroughly. They made the picks, and have put them in their schemes. They watch and make judgments(opinions) on these guys almost everyday. Then they watch a guy like Gurley, sees what he brings to the table, and how he can help the team win. He has 'burst', 'vision', 'strength', etc.. all these attributes are what he currently posses as a player in his prime. There is no sugar coating what abilities he possesses; that's why he's here and why he starts. They don't go back and forth discussing his attributes because of a bad read, dropped pass, busted assignment, or even a bad season. They know what he does and does well, and until he starts showing decline in particular areas of his game consistently, there will be no changing of who he is as an NFL player on any team.
But for some dadgum reason, fans all over the globe continue to question the abilities of players; over and over, week to week. It ends up becoming monotonous and a beating of the dead horse. In this regard, it's a negative because all you're doing is stating an opinion that you've already stated before and that others have stated. 'Let's start splitting the reps', or 'let's find his replacement' are constant phrases used when a player doesn't perform to a fans liking. The fact of the matter is this: 10 other players have to do their jobs somewhat successfully in order for another particular player to succeed (whoever, or whatever position they play). If they don't (coaching staffs included), does that mean that a guy like Gurley is in decline or has lost ability?
Come on, man. This dude is a stud and there is no denying that. I've said it before and I'll say it again,"Players on the other team are paid professionals too." It's about matchups.