The move doesn't surprise me because at this point in his career he is at the top of his game on and off the field, and I don't blame him for taking advantage of that.
Root is already having back problems, and that has hurt a number of players careers through the years like Atherton and most recently Michael Clarke. I hope it won't be that bad that it stops him bowling or hinders his batting, but if there is potential for it to be a long term issue, then cashing in at this point instead of being "loyal" would make sense. After all, a bad back could shorten his career significantly - Atherton retired at 33 and Clarke at 34, without their back problems it is likely they would have played on longer.
But even if his back isn't going to be a long term problem, cashing in at this point still makes sense because we've seen batsmen have these great periods in their careers only for them to hit a rut or slow down. Or there is always the potential for that career ending injury. So setting himself up financially is a sensible idea whether there is a concern over his back or not.
In an ideal world it would be nice to have seen him stay with the brand that has been with him since he was a kid. But at the end of the day he's made a financial decision and hopefully it pays off for him.