4th rounders are not shoo ins, and getting a developmental QB who is still very young with the skills of a 1st rounder is not an unreasonable trade off.
Remember what round Dak was drafted in? 4th ... lol. And he wasn't considered a shoo in by any means, plus he wasn't expected to be anything more than a 3rd string guy for a few years. .
The only notable knock on Trey Lance coming out of college was just that he didn't have an ideal amount of experience, but he was rated as having a high football IQ and great size and athleticism for the position.
As for Dak not going anywhere, that's probably true, but there is still a lot of value in a high level backup, and if he eventually proves to be capable of more than that he would become a great trading piece for the Cowboys.
Good teams don't ignore QB's in drafts even if they already have a good QB. Even with Brady as their QB, over the years NE still drafted QB's, even using 2nd and 3rd round picks on them