they are all pretty good, not claiming any are elite, just solid, and much better than what the Bears are trotting out there in their secondary. You are right however, the Lions did have one of the worst secondary's in the league, so upgrading it to respectful is a big step forward. As for St. Brown, that is almost like asking what happens is Cooper Kupp goes down, it would be a huge loss, but hoping Jameson Williams, now healthy, steps up. Plenty of DB talent still to be had in the 2nd and third rounds, the additions now mean they don't need to add one in the first.
My dream scenario is the Lions still acquiring Bobby Wagner in FA, taking Tevon Wilson @ 6 and Bijan at #18, then BPA the rest of the way out.