First if all, we didn't "lose" Pyne, he left of his own accord. While this was his first year as a starting qb, it was his 3rd year in college football and he offers, at best, a pretty limited skill set. He is as immobile as Coan and was not a good downfield passer because he simply doesn't have the arm to pull it off. Neither of those two things are going to get any better. He played hard and as well as he could but we aren't ever going to rationally compete for the playoffs with him as the starting qb. That doesn't mean I would tout Buchner as the be-all, end-all, but he is a lot closer to a qb that can lead to the play offs because he has excellent mobility and a strong arm than Pyne. Buchner's upside is a lot higher than Pyne's. Whether Buchner ever fulfills that is left to be seen but it is likely that Pyne has already reached the top of his capabilities.
Pyne likely left because he saw the handwriting on the wall. Folks seem to forget that Buchner beat him out and anyone who logically watched the Gator Bowl saw why. With Minchey in house and theoretically Carr joining in '24 much less the presence of Angeli if he sticks, Pyne was unlikely to ever play for ND again.
Hartman is perhaps the best qb ND has had in the last 10 years. We'll see how it plays out.