Great deal for both sides, this seems to be the new way of operating for clubs. The Angels lock up the prime years and get a slight bargain on a short-term FA deal which they had no prayer of getting Trout to sign once he hit the open market, and Trout still sets himself up for another huge deal down the road.
If Trout hits the FA market at 26, he is commanding an 8-10 year commitment at some ungodly AAV. Getting him signed for his age 26-29 season is a coup for the Angels no matter the dollars. Meanwhile, Trout gets paid upfront now and doesn't have to navigate the arbitration process year to year. He is assured security for life should his career (god forbid) go the way of Sizemore or Griffey, and this won't prevent him from still getting a record setting deal once this one expires.
Nice compromise for both sides and great for the game of baseball. Fail to see a downside.