It's an interesting argument to pay by WAR, but I don't think it's a very healthy one for the game. I think better would be to have fixed salaries for young players (maybe 1M?), with additional (potential) position dependent bonuses paid separately for both offense and defense compared to the league average at the position, perhaps prorated to the number of games played at the position and adding an additional kicker for playing multiple positions. That would encourage young players to focus on both sides of the game, reward them for performance while also providing some cost-certainty for the team (or at the very least the team knows they'll be paying for performance). For years 25 through 27 the fixed portion can increase each year, and at 28 there's automatic free agency.