Diversification marks the way forward...
It may sound glib, but one Railway Model seller I know that has diversified is Richardsons in Feltham, Middlesex.Although primarily a Builders' Merchant and Timber Suppliers, they actually hived off a section of their business premises to sell models. Not just Railways either, but lots of variety too. What could be easier than buying your baseboards, support frames, wiring, and your track etc., without having to drive miles, or wait for something to be delivered 'whenever'.
Maybe not all vendors have the same sort of premises size-wise, but it has enabled them and some other businesses not to have to rely on only one type of income.
On the subject of specialists however...if I need parts to build certain Locomotives I have only a couple of sources, and they certainly don't seem to be suffering, but as we all seem to agree...specialist knowledge is worth paying for. The follow-up to that is how long will they keep going , if youngsters don't pick up the challenge?