That's the thing about those long term contracts, you have to perform, year after year, for almost the entire contract for it to be worth it. Its hard, baseball is a tough game to be consistent. One or 2 injuries, and long time on the IR kills it. You have to play.
All the odds are against you on long term deals. Anything 5 years plus after the Age of 29.
10 year contract work, if the Player is say 23, and its over when his best days are behind him. Betts just turned 27, he'll be 28 when he is a FA, too old at that time for a 10 year contract. You will hopefully get 5 very good years, maybe. Then contract catches up to the team. 5 years at huge money for a declining player. That blocks others to play his position. Always the DH I Guess, if your still hitting good.