Instead of waiting for a semp to rot out, I pulled a semp rosette that looked like it was in the beginning stages of rot. The roots looked healthy but the leaves were starting to go. Here is what I found:
This is not a great photo. My camera can't focus any closer but I think you can see three white spots on the soil taken from the plant. I think those are root aphids. They are less than 1/16 inch in length.
In contrast to what I found on the internet about root aphids, these are not especially triangular in shape (instead longer) and they move fairly quickly (but I understand there are many types of root aphids). They are white and the right size.
Also, in looking at the semps affected, I realize that they are all in the middle third of the bed so proximity is also a factor on which plants are affected.
Now, I have to figure my next step.
John