Bundy has it right again.

Deuce will have at best 1 season left in the tank. One. Next year we will be picking 32nd, and we won't be able to get Deuce's ultimate replacement. Besides, has any of you considered that RB is the least risky pick you can get when you're drafting this high?

The real question is what's in Payton's mind.
It doesn't matter what this forum thinks. It’s already been said that Payton loves Mendenhall (no link but he said it) and the pick does secure he legacy as a great offensive guru. (see picture below) Payton wants an offense that creates mismatches and he needs a primary RB/weapon. With this, he and Drew will be able to once again dictate at will.

Thomas should be used as the primary backup and if you love Deuce, you should realize that he should retire before he ends up a cripple in 10 years.