When the DH first came into use, it did extend a lot of careers, and you did see a lot of guys move to DH their as their gloves got worse, but that was in an era where everyone who made the bigs made them as a position player somewhere. Since then, the DH has been available for players who from the beginning of their careers were poor defenders but hit well enough to hold down DH, so the profile of the DH has shifted. Sure a few guys still end up there when they lose their fielding (Cabrera) but as many or maybe more start there - JHM for example (assuming he were to return to the Tigers and stick as some point). The availability of younger no-glove players, who are lower cost, means holding on to your old players as DH's is less attractive then in the first generation of the DH.