Betts has literally no impetus to take an extension now beyond perceived market value. He made $10 mil last year. He’s making $20 mil this year. Even if he has a horrendous, injury plagued 2019 season, the Sox are going to hang onto him for 2020 and the lowest he can get in arb is $16 mil (20% drop from prior year). He’s essentially guaranteed to make $46 mil from 18-20 with a strong chance of something bigger in 2020. He’s now made life altering money. The next paycheck is a legacy check. It’s not just about him, it’s about the players after him. He owes it to the other players to maximize his worth. This is why you try and extend players like Beni when they have sub $1 mil deals in place and you can guarantee them a windfall in exchange for trading in a year of the FA clock.
Think about this. If you’re a professional baseball player, playing a thousand miles from your home, what does it matter if it’s in Boston, NY, Philly, Toronto, etc (outside of taxes). And while yes, the amount of money is ridiculous, but the more you make during your prime, the even higher the amount you’ll have when you’re done. It’s capitalist to take the money, especially when you’re already guaranteed a ridonculous sum.