Either way the times are different and QBs drafted in the first round are almost always required to start especially cause of the media and fans want them too. We were winning and the Crowds still demanded Manziel to play and so did our FO. Your argument is solid other than the fact QBs back then were draft based on where they should be not like in the current era where we draft QBs based off of potential and not how NFL ready they are. In that era there wouldn't have been a single QB drafted in the first round nor asked to take snaps in his rookie season if you look at the differences. Teams only will keep veteran QBs if they are at elite levels still otherwise they are cut for a rookie with potential. The amount of busted 1dt round QBs lately had been a lot higher than possibly any other era.
Also there is a huge difference in the last ten years when it comes to QBs, most top QBs in the draft start day one and they don't always pan out. With teams not keeping those veterans the young guys are forced to start and many before they are ready hence why we are seeing a ton of turnover in the NFL at QB. For every Manning and Ryan there is Gabbert, Ponder, RG3 type of guys. Not everyone can start right away and this current era says you have to start. Look at Goff last season, the coach wasn't planning on playing him, which isn't a wrong thing since the team wasn't good.
Wentz people forget was part of a pro style system and could do everything needed to play in the pros. Out of last year's class, he took the least amount of snaps in the shotgun and actually played under center a fair amount. He didn't have any concerns like Kizer does when it comes to the mental aspect of the games since in college watching his film, he seemed to often times throw to his check downs if he felt the big play wasn't there. Kizer made a bunch of boneheaded plays in college and was inconsistent with a lot of the aspects of his game. Wentz was ready to play in the pros, Kizer is far from it. Kessler was ready to play in the pros last season, on a Madden stat sheet if they were being sincere his throwing power and deep ball accuracy wouldn't be all that high, but his awareness, short pass accuracy and his mid throw accuracy will all be solid if not on the higher end. Now injury would be lower than I would like from a starting QB, but that's life.