I may be the odd one out but, I don't think the new players have much if any bearing on on Jordy and Cobb needing to be extended before the season starts.
You don't want to replace your best players. You want to upgrade the depth. Improving the team isn't getting rid your top WR. You upgrade your worst players and see if they can move up the depth chart.
You can't replace players with a promise of potential. Lots of players have had potential and never amounted to anything. One or more of the new guys could just not pan out or end up like Terrance Murphy. In fact the odds are probably pretty high that at least one of them doesn't work out in the long run.
The new WRs are all going to be on rookie contracts for about as long as I would expect Cobb and Nelsons extensions to be. By the time they are ready for new contracts, we will know if they are worth keeping over someone else.
A big factor to me is that the Packers offense is pretty complicated. WRs tend to take 2-3 years before they really catch on. If we let one or both of our experienced WRs go, that could be a huge set back for our offense.
If we don't hang on to Jordy and Cobb for next year, and the one we keep gets hurt, do we really want to go into a season with Boykin, Adams, Abbrederis and Janis?
I think we pretty much have to extend at least Nelson and Cobb. Preferably before the season starts.
I want to go out like my Grandpa did. Peacefully in his sleep.
Not screaming in terror like his passengers.