It's a big risk that Texas can afford to take, and they had tried to offload Kinsler in the previous offseason with zero takers (he rebounded with a very good 2013 after a poor 2012 that made a trade more tasty for another club). They still have Moreland (a good defensive 1B) and perhaps could use Prince as a DH if necessary, albeit a hefty pricetag for a DH. Texas has a big advantage for left-handed hitters with the RF/RC porch and circular air flow. They're now rumored to go after Choo albeit he might cost more than they would prefer. He does get on base frequently and may be worth overpaying to plug in the leadoff or second hitter spot.
Another way of looking at the deal is Kinsler will earn $57m and Fielder $96m over the next four years. Detroit is paying $30m of that, assuming all in a lump sum or structured over x years, greatly reducing the financial impact. Kinsler makes $16m in each of 2014 and 2015, $14m in 2016, Fielder makes $24m each year. If Detroit is kicking in $6m per, that means Texas is paying $18m each season (for first five years out of seven), only $2m more than Kinsler ($4m more in 2016) while getting more power in the lineup and opening up 2B for Profar. It's when Fielder gets to the back end of the contract the true cost will be felt, especially if/when he's hampered by injuries and/or declining overall (and I do take into account that fat body will eventually break down).
The 4% run production is not taking into account Texas lost Hamilton and Napoli, and Cruz was suspended for the last 1/3 the season, greatly reducing output compared to previous seasons. They also lost the LH power that takes advantage of the RF porch dimensions ('326 down the line, '334-381 along porch). Just like climate change, it's a period not a sole season/year that reflects a true picture of ballpark advantages as well as the hitters in said lineup.
A ballpark overlay shows Comerica has slightly shorter LF & RF dimensions but a much deeper LC, CF, and RC. The design of the park allows wind to enter and circle around which allows fly balls to carry further, resulting in more RF/RC/CF homeruns (and hits) than under other conditions would normally allow.
http://hittrackeronline.com/ballpark_overlay.php
http://en.wikipedia.org/wiki/Rangers_Ballpark_in_Arlington#Field_dimensions
http://hittrackeronline.com/stadiums.php and
http://espn.go.com/mlb/stats/parkfactor
Year - Park - HRs - GP - Avg - RS factor
2013 - Comerica - 153 - 81 - 1.89 - 1.139
2013 - Arlington -159 - 82 - 1.94 - 0.985
2012 - Comerica - 159 - 81 - 1.96 - 1.071
2012 - Arlington - 202 - 81 - 2.49 - 1.183
2011 - Comerica - 158 - 81 - 1.96 - 1.061
2011 - Arlington - 228 - 81 - 2.81 - 1.409
2010 - Comerica - 141 - 81 - 1.74 - 0.981
2010 - Arlington - 174 - 81 - 2.15 - 1.091
2009 - Comerica - 179 - 81 - 2.21 - 1.026
2009 - Arlington - 215 - 81 - 2.65 - 1.085
2008 - Comerica - 202 - 81 - 2.49 - 1.077
2008 - Arlington - 204 - 81 - 2.52 - 1.142
2007 - Comerica - 187 - 81 - 2.31 - 1.051
2007 - Arlington - n/a - 81 - n/a - 0.979
2006 - Comerica - 161 - 81 - 1.99 - 0.980
2006 - Arlington - 179 - 81 - 2.21 - 1.081
8yr avg - Comerica - 1340 - 648 - 2.07 - 1.048 (8.386)
High 202, Low 141, Avg 167.5
7yr avg - Arlington - 1361 - 568 - 2.40 - 1.119 (8.955)*
High 228, Low 159, Avg 194.4, +13.75% HR, +6.3% Runs
*8-year average for runs factor but only 7 for HRs due to lack of data for 2007