It's a fair contract in my book.
It is just the wrong moment in the team's development to be signing a guy like Nick Young to that type of deal so it's a bad deal despite the money being fair.
I think Nick Young is a player you only sign to a substantial multi-year contract when the rest of the team is in place and you have exactly the right type of circumstances for him to succeed in. Sort of like Marcus Thornton. Two very similar players. Good scorers but do not do a lot else. Not a one size fits all type of player. Need very specific situations to succeed as a result. On majority of teams their playing time will be limited due to flaws. On small number of teams, generally top notch defensive teams who have stars who are not great scorers and as a result team relies on supporting cast for needed offense, a guy like Nick Young or Marcus Thornton can be quite valuable as scoring orientated role players. Starting SGs who can get you 16-18ppg.
Nick Young is NOT a "good scorer" is the problem with your entire premise. He is an average scorer who happens to shoot the ball a lot. There's a difference. Look at his shooting splits; they are all hovering right around the league average. Combine that with the fact that every other number of his across the board is below average, and you get a what is called, in technical terms, a "****ty basketball player."
Nick Young tends to be a bit higher than average in terms of scoring efficiency when you include turnovers into the equation. Low turnover player due to style of play.
Even at average scoring efficiency, I think players who can reliably get their own shot and score 15+ppg (while offering little else to team) have value to teams. They are not stars. They not guys you build around. They are support players. Role players. They are not valuable everywhere. Only in certain situations.
Sometimes ... in the right home, they just add more than a basic "3 and D" player. Sometimes teams who are desperate for scoring and cannot afford to get a star player to do it, are better off with somebody like Nick Young than a defender/shooter type.