Dexter does make a line of axles called Nev-R-Lube that use sealed bearings. However the design is totally different than standard bearings and hubs and would require entirely new axles (as new spindles are needed and they are welded into the axle tubes) and also new drums and hubs.
GOOD QUALITY tapered roller bearings and quality grease and regular maintenance will give you millions of incident free miles, and the rare on the road failure can be cured many times with a new bearing and race. The Nev-R-Lube hubs require a little more expertise in the bearing replacement and the odds of finding replacement parts are nil without a long wait. I'll stick with regular old bearings. I use USA made Timkens and good synthetic grease with Molybdenum in it.
The Nev-R-Lube bearings are also sensitive to side loading and can be damaged by curbing, or even the use of wheels with too much offset.
https://youtu.be/lKJo3rgN1cM
https://youtu.be/EQwMjcvPr7I
Charles