The grease that was available in 1939 would not have lasted long enough to warrant using sealed bearings.
Many VMC's have grease spindles that run multiples of that a J head can, for many years.
high quality bearings made now are many times better than they were even 40 years ago[supposedly better vacuum furnaces, less carbide spots on the balls or so I am told]
If you were designing from scratch today, would you choose dirt or no dirt?
All that said, the original bearings last a really long time.
Having had to repair many "improvements" to machines in the past, Im a big believer of "if it aint broke, dont fix it." Ive seen many very high hour Bports (and other knee mills) that still had perfectly good bearings despite not being sealed. My personal take on sealed bearings - if your knee mill is in an dirty/dusty enough environment where they need to be sealed, youre going to have other serious issues besides the bearings.
Would be interesting to know how Moore and Pratt Whitney lubricates their jig borer spindle bearings.