I don't post much in GD, but I'll give you guys my take on Rivers.
His arm strength is the same that it always was. His decision making is still the same. Two things have changed. His technique and his weapons.
His offensive line gives him to room to step up into the pocket, and his targets aside from DX have not been able to create much separation. Rivers is one of the best at climbing the ladder, and throwing in tight spaces. However, if he can't step up, then he can't make the same throws he has been known to make.
2nd: His technique. Most QBs generate their throwing power, through 3 motions. One is to set the feet, and throw leaning forward off the front foot. If you have pressure up the middle, that is scary for a QB. Two is by rotating your hips, your body follows and increases your arm speed. Three is arm strength which I already said hasn't changed.
So what has: Rivers isn't setting his feet on every throw. And because he has less time in the pocket, he has less time to start his throwing motion, which means he doesn't get full hip rotation, and his throws lose power. Then the fans think his arm stength has declined.
IMO, that is the biggest reason Norv and AJ had to go. AJ failed to rebuild our OL, and Norv failed to correct the sloppy fundamentals that Rivers had developed. When you are termed a QB guru, your QB should play with perfect technique. River's hasn't these past two seasons. Whoever comes in, Arians or Roman hopefully, will correct that. And Telesco is obciously not afraid to dedicate an entire draft to one side of the ball. Let's hope he dedicates a lot of this draft on our OL.