About 12 years ago, Jim Kibler showed up at the NMLRA Gunsmithing seminar with a bunch of his first kit guns for the students of a class he was going to teach that week. A class on building one of his kits. A couple of us custom guys were standing around as he was unpacking one of the kits. Jim asked us what we thought of the kit. I was admiring the fit and finish of the stock and inletting, done by a machine and feeling a little intimidated I popped off; "Well dang Jim, if you shook the box I think the rifle would build itself!" I couldn't tell if Jim was offended or appreciative of my comment.
Everyone who showed up to that first class of Jim's at the seminar were new faces, newbies to gun building. These were people who would have never dreamed of signing up for a class to say build a Jaeger from a pricey hunk of English Walnut and a pile of crude brass castings. Each year there after at the Gunsmithing seminar, there was Kibler kit gun class. And, each year there were new faces, students in the class. Each year a few of those guys in that class would drift around to the other classes going on and watch what the plank/custom builders were learning. I have to believe a few of those guys have moved on to custom building though I don't know for certain.
In my area, there remains a fair number of people who want custom sidelock firearms built in period correct fashion. A good example here is I am currently finishing up a left handed fowler in Black Walnut for a customer, it has 1/4" of cast on and a length of pull to the trigger matched for him along with a few other specific features requested. I am not aware of any left handed kits or production guns out there. So, the left handed shooter has to come to a custom builder to get what he may want in an antique firearm build.
So, to answer the question are availability of these kit gun reducing the number of people willing and able to build a custom piece from a pile of parts and a board? IMO No. The numbers of people willing to spend say 300 hrs. to build one of these guns is dropping yes, but that is happening for some or all of the reasons already mentioned by others, not the kit guns themselves.
I am seeing problems for the custom builders from areas of greater concern than the kit gun market - inflation that has more than doubled the cost of parts for a build in the last 4 years, government regulations that are driving foundries that make some of the parts we need out of business, looming regulations that could result in a ban on lead both in the projectiles we use and in the parts for building just to name a few.
As our society and economy change, all forms of hand craft, art, which is what these guns really are, is going to become harder and harder to come by. Kudos to all on here who are fighting the head winds and keeping this art form alive.