Hi, that was just my personal opinion, take it or leave it........the top bearing is there to ensure concentricity of the spindle under cutter forces that apply a side loading to the bottom of the spindle and a corresponding opposite side deflection to the top of the spindle due to it's leverage over the length.......it can pivot in the bottom angular contact bearings when loaded, etc.
As you were working on fitting a top bearing to the spindle I suggested that you "could" have fitted two SAME SIZE bearings, one on top of the other, in a deeper cup housing and so increase the loading capability of the spindle at that point.
The force of the belt will impinge on the bearing oil/grease film and thin it out thereby making wear in the bearing an increasing factor, precisely where you don't want it.
As you've finished the job it's now merely academic, no more to be said.
Ian.