In my experience with SLN trees and Antonovka rootstock, it takes a couple years for the roots to get established. Not a lot of growth at first. But when they kick in, they really take off. Antonovka roots look more like carrots than the hairy, massed roots of other rootstocks. But they turn out fine.
When Bill MacKentley owned SLN, all the trees I got were over 3 ft. tall and averaged 1/2" caliper when shipped. Most were whips - only a couple had a branch or 2.
I think maybe the new owner, Connor, ought to consider keeping some of his "high demand trees" to grow another year. If folks are going to abandon SLN because of tree size, he might be further ahead in the long run. Graft more of the popular crabs than the usual number, and keep part of those for an extra year. Keep grafting larger numbers and letting them get size to them until he gets ahead of the curve. He could also charge less for smaller trees in the meantime. But he should send out a letter or e-mail explaining what his plan is for getting some "banked" trees, so they're larger.
Blue Hill has a good selection of trees on good roots (Dolgo) and others. His trees are great looking. I have to wonder if his trees are all selling out each year, if he'll find himself in the same boat as SLN - not being able to keep up or get ahead. Trees HAVE to have time to grow if people want bigger, feathered trees when they buy them.