It sounds like there is more going on than just a lack of mobility. If he stays in A with a runner on first, he is foregoing a positon that would require less movement to keep up with runners. That's counter intuitive for an umpie with mobility issues.
So it sounds to me like a combination of lack of training and lack of mobility with a dash of indifference thrown in. (cell phone)
I'm sorry that you had such an experiene with an umpire.
In my association, evaluations are based on performance and include consideration for mobility and reaction to developing plays.
We have some umpires who were once top level, and still have excellent judgement and mechanics knowledge, but for one reason or another have lost their ability to get to where they need to be and when. We love them dearly and appreciate what they have contributed, but we reduce their rating and the level of games when this happens.