To be honest, I am tired of talking about past QB that we "ruined" Much of what a QB is able to get accomplished isn't based on when you start him as much as is he ready? Is he being coached up properly? Is the OC going to do a good job easing him in the system? The mental part is there and he has physical ability, for me its reps with starters in practices/preseason games that will help decide where he is at.
Coaching, readiness, ability, mental frame of mind, with the speed of the game and so on. I can go on but there wont be an answer to these questions till Josh Allen plays with #1's in a preseason game. you mention QB's that failed because of being started right away? what about all the good QB's in the league that DID start right away and are great starting QB's in this league at the very least?
Foster gave up on a long pass play that would of been a TD, not counting dropped passes and O'Line penalties that were dictated by a weak third string unit. To be honest with you I doubt that coaching staff even fully knows where Allen is at till he plays with starters.
lets not pull out our cute little crystal shiny balls and look into the future, lets ENJOY the process and ride with it.