First of all Upton does NOT have to opt-out to negotiate a new deal. That is false. He can do anything between now and the time he has to make a decision right after the World Series ends. If he does negotiate a new deal the old one is superseded and the new one is enacted.
Secondly there may be teams out there that are willing to pay him more, certainly. But Upton and his agent have to weigh the value of him opting-out period and whether he can really gain more money on the open market and more importantly where he will have to play/move to make that money. Maybe Justin does not want to move to Philadelphia and play there. Geography will probably play a role in his decision.
Tax rates will also play a role in his decision and the Angels. If they renegotiate his contract it will fall under CA tax rates and the Angels will have to pay him more for him to make an actual increase in salary which the Halos may not want to do. I could be mistaken here but if he keeps his current contract it falls under Michigan tax codes (someone well versed in baseball contracts will have to correct me here if I'm wrong) not CA.
Finally I think that Upton would prefer, based on his family living/working on the West Coast, to play on the Best Coast. That would via geography give the Angels a slight advantage. Playing with Mike Trout might give the Angels a slight advantage. Playing on a team that is about to get significantly better might also be a slight advantage.