How do you define an established starting QB? Was Brady an established, starting QB? How about Cousins in Washington? Brees in SD? Smith in KC? Kap in SF? How about Rivers? Is a "bus" driver an established QB?
If I don't think a QB can win a SB, I move on.
I guess you missed that part of "age/injury" withstanding.
Brady is well into his 40's now is he not? And he really didnt perform all that great last yr, especially compared to previous years.
The Chargers drafted Phillip Rivers to replace Drew Brees, he was already there. They drafted Rivers in part due to long term concerns over a Brees injury situation (seemed like it was a throwing elbow thing)
The Chargers are moving on from Rivers now due to age and cap value. He too had a year where he wasnt considered as good as previous years. Rivers also hates what the Chargers did moving out of San Diego.
The only one you listed that MAYBE you have a point with his Alex Smith. I think he's comparable to Prescott as a player in some ways. However even in his case, you had a guy who had been in the league 13 years and Andy Reid had coached him directly for 4 or 5, and had seen everything he could do. That combined with his age made the Chiefs decide to get their QB of the future. Prescott has been in the league 4 yrs and McCarthy hasnt really got to coach him 1 day yet.
And please dont mention Kapernick, the dude was never a legit QB and that's why in a league in full desperation mode for quality QB play, he cant get anyone to give him a shot. He's a turd who also happens to be a terrible passer.
I agree with your last statement though, once you know a guy isnt capable to get you over the hump, you move on. I kinda doubt McCarthy has had any chance to make that decision yet although I strongly suspect if he didnt have confidence in what Prescott could do, he either wouldnt have came here at all or insisted they start thinking about alternates pretty quick.