Around here, ( Calgary ) they still seem to be useful, or a nuisance depending on your viewpoint. Still a fair number around, though they seem to be getting scarcer. I've notice that a greater percentage of those left have started using cargo bikes or trailers of one sort or another. One of the courier companies had even started sourcing their own utility trailer. ( Might ask where they get them, looks useful ) I don't know if this is simply the individual courier looking to increase their revenue, or if it's being pushed by the companies.
I wouldn't expect it to be a great money spinner, if you're good, and efficient, you can make a living at it, but you're going to have to work all the angles, do your own maintenance, claim everything possible against tax, source cheap parts etc.
One reason I have toyed with the idea myself, is that in Canada, you can claim $17 of food everyday you work, as fuel. I kinda like the idea of having your food as a tax break.