Fielder had an off year, but he is 29, and if you look at the batted-ball stats, his line drive rate was actually a lot better than it was his last 2 seasons in Milwaukee. There is some good evidence that the bat speed is not diminishing. There is a huge risk taking on that sort of money - but the deal makes all sorts of sense for the Rangers in the short term. $30 million bucks, an upgrade at 1B, an even bigger upgrade at DH if they go that direction with Prince (and they really ought to), and finally giving them some options with Jurickson Profar.
Detroit it improves their defense a lot, by shifting Miguel Cabrera to 1B and putting Castellanos at 3B. Also frees up money to extend Scherzer. Kinsler's bat should be fine at 2B, though the plunge in power the last two years raises eyebrows but at least cheaper eyebrow raises than Fielder's.