Marshawn Lynch would have been great, but that's been covered over and over. Seattle had a higher pick to give up. Additionally, in what position would the Packers be salary cap wise right now even if they would have gotten Lynch? He is set to make $8.5 million this season, $7 the next, and $9 the next (http://www.spotrac.com/nfl/seattle-seahawks/marshawn-lynch/). I believe he only had one year left when they traded for him. It was definitely a risky trade in many regards that just happened to work out. Having a salary like that on the books would no doubt complicate things otherwise on the Packers roster. Additionally, his stats have been great, but it's a completely different team and system. Obviously he would be an upgrade in talent, but it's very unlikely he would have the same stats on the Packers roster. Again, the bigger problem comes down to the emphasis on the run game and balance, as the Packers were having some success late in the season when they actually ran. It would take a lot of convincing for me to believe Lynch would be worth almost $9 million/year given all the other players that need resigning.
Trades and player acquisitions just aren't as simple as they appear on the surface to us fans. There's so much that goes into the transactions, and a lot of the considerations are 2, 3, or 4 years down the line. I'm sure Ted Thompson would love to sign many other players. However, he has a budget to work with that is well studied and thought out beforehand, and he can't/won't go beyond that; and he shouldn't.