It's much easier to start a guard as a rookie than it is to start a WR as a rookie. That's not the greatest comparison. There's not as much of a learning curve.
I do agree that the draft is deep in WRs, but like you said, few WRs make an impact their first year. Because I think Watkins would make a big impact his rookie year, I think it'd be worth the trade if the front office decided to do it.
Who would you sign in free agency to be our #2 WR? The WR class in free agency is pretty poor this year. Decker, Nicks, etc. Decker is guaranteed to get overpaid because of the numbers he put up with Peyton. Nicks is very inconsistent and often injured.
I'd be happy if we drafted Gilbert at 10 (it's what I think is currently most likely). We could also draft a guy like Dennard if we trade back. But I'm not too interested in grabbing a mid-round CB. We have enough projects already. Slay, Bentley, and Houston should improve, Slay and Bentley because of experience and Houston because he should be healthy. Plus we can see if Green and Greenwood are ready to step up. CB is definitely not a strength, but I think having Teryl Austin as our DC will help them develop.
Also, I think we can address safety either through the draft or free agency. There are several possibilities in the draft like Haha Clinton-Dix, Calvin Pryor, Ahmad Dixon. If we go the free agency route we could sign Chris Clemons, James Ihedigbo, Donte Whitner, Antoine Bethea, etc.
All I'm trying to say is that there are a lot of different ways to things and trading up for Watkins should be considered, even if it isn't executed. Like I said above, I'd be very happy with Gilbert at #10. But I'm a little worried if Gilbert is gone before the Lions' pick because I don't like the options that leaves us with, unless we're able to trade down.