It would be the height of idiocy to take a so-so QB prospect now, with another 4 years left on Dak's deal. It'd be one thing if it was an amazing opportunity with a guy who falls, but Ewers in the 2nd is just kind of meh.
But the real idiocy of it is that you have 4 years left on Dak's deal, the rookie would have a 4-year deal as well, so you're in a position where you basically have 0 years of a rookie QB contract to give you max cap flexibility.
If the rookie works out you're going straight from Dak's deal (with whatever dead cap) to an extension for the rookie.