My ill-considered and poorly thought out opinion:
There is a particular generation of cyclists out there at the moment for whom a steel frame represents something important. They have grown up wanting one, but have settled for aluminium intially for a price/availability reason.
The advent of carbon frames may have hoovered up some of their desire, but that custom steel frame is still seen as desirable by these folk, who now have the cash and the insight to get one.
As cycling has become aspirational, maybe even middle class, perhaps the new golf, so the market for steel and custom steel has been maintained.
…but…
This desire is confined to a particular age range, despite the retro appeal of steel frames, and so I suspect that the market for such frames is already well supplied. If you are one of the old guard, with a history, with a track record, with the kit and the connections, you can do ok.
But I suspect that it is a changing scene, and as the steel is real boys get what they want, the market for custom steel will contract.
Not a viable business proposition in my honest opinion, more of a hobby.