I'm not so sure his defense declines all that much. Experience counts for something, and remember, Betts has only been an OF'er for a few years.
Did Dewey's defense decline as he aged?
Sure, part of Mookie's great D is tied to his speed and quick reaction time, but that doesn't mean he can't improve in other areas to compensate for that projected decline. He's also a smart base runner not just a fast one.
Also, where is the evidence that quick twitch muscle reaction time declines sharply with age? Is there another player in history that had this quality? I'm not sure they even measured this long ago, so where's the evidence?
Another possibility is that he loses some speed and twitch reflexes but gains in power. There are many examples ofplayers who hit 20-30 Hrs in their 20's then hit 30-40 from 30-35 or later.
I'd pay Betts more than $300M x 10. He'll likely be worth $37.5M x 2, $35.0M x 2, $32.5M x 2, $27.5M x 2 and $25.0M x 2. That's not even counting on rising player salary averages over the next 10 years.