I did a complete study a while back, and a clear majorty ended up being bad deals for the team- many were horrible, some like Manny worked great but still ended badly.
I realize the risk. I just think Betts has so many skills, and his unique "twitch skills" makes him very special. There's no "history" to compare him to, I mean someone was comparing him to Tim Raines and Pudge Rodriguez, just because he's kind of small. Come on! Betts is not Raines or Pudge.
Maybe his defense will decline due to slowly losing his speed, but he has tremendous instincts and a strong arm. In Fenway Park, that is and always will be a huge plus. His value will not be greatly diminished by reduced SBs. One can imagine his power increasing as many players do, once they reach the 29-34 years.
He also brings fans to the park and TV. He sells jerseys. He may not be worth $30M his last 2-4 years of a 10-12 year deal, but he may very well be worth over $50M for the next 2-5 years.
I realize I may eat my words, and things could go horrible wrong, but to me, Mookie is who makes the Sox the Sox.