Come on, how can you make this argument? "Running to cover the bag". A good many of them are waddling to cover the bag and if they are holding somebody on they are already there. Think Prince Fielder could play 3rd, Frank Howard, George Scott, Jim Thome, Frank Thomas who actually DH'ed finally more than he played first, on and on and on. Have we not seen players time and again tried at 1st in order to improve their longevity and keep them able to come to the plate? Seen that much at 3rd? When people talk about ways to keep Ortiz in inter-league games do they talk about the possibility of putting him at 3rd or at 1st?
Once again just like the issue of the possibility that Youk might have injured himself batting being just as strong as fielding, it does not matter whether his injuries to date can be traced to throwing. 3rd is less taxing on the body than 1st.
In addition, especially for baseball players it is the accumulation of dings and knocks and pulls and strains that occur over the course of the season that eventually makes things like diving right, taking one in the chest and then scrambling to one's feet to make a throw that much more challenging. Those players are not going to back off even if they should. They are going to try to make that play even if they are already nursing some injury that makes that play much more difficult. The result sometimes is that they injure themselves further because the overtax one part of the body to protect the injury or injuries. In fact another opportunity that is often discussed relative to moving a player to 1st base is a player coming off of injury....why....because it is less taxing with more opportunity to protect the healing/recovering body.