You're correct that we've completely botched the development of Sam Darnold, but sometimes it just doesn't work that way.
If there is a quarterback we think is a can't-miss quarterback in the draft, and Darnold has a mediocre season, it will be hard to pass them up. Yes, you don't get the full picture of Darnold at the end of this season, but that's just another consequence of the poor development.
If we get the #1 pick, Trevor Lawrence is the pick. That will either have meant Darnold got hurt or sick or played poorly. Darnold would still be on the roster next season. We could either trade him right away, or we could keep him as the starter and tag-and-trade him after the season. We still would have a full complement of draft picks on top of that pick. Yes, there is a chance Darnold could break out in year 4 or later, but if he's bad enough where we get the #1 pick, the odds are stacked against it, and it happens elsewhere, we lost out. It still worked okay for the Chargers with Drew Brees and Philip Rivers.
Hopefully Darnold bounces back and shows he's the long-term answer. I still have faith. But it's not as steadfast as it was.