I am not overwhelmed by the 2018 draft class of quarterbacks.
I think teams like New England, the Giants, the Packers and the Saints laid off trading up to get Mayfield, Rosen, Darnold and Allen for the same reason. That's not to say these guys can't be solid NFL quarterbacks at all.
In the case of the Patriots and Giants, with each having an old quarterback, it was evidently decided to build around Brady and Manning with adding to the OL and getting a stud running back.
The Giants have Davis Webb from last year's draft out of California. He never played at all in 2017, but has potential. I think the Giants are hoping to develop him under Manning in the next 1-2 years rather than address Manning's age now.