I am not an expert on the market. That said i do believe that SxS guns are a niche market. Those of us who like them frequently like them too much.
Further I think the following and I believe it common knowledge.
Smaller gauge vintage guns are in shorter supply than 12 ga guns, they are more desired in the market so they simply cost more. I do not believe the demand for good condition small bore side by sides is significantly down. Or that the prices are depressed. The occasional good deal on a small bore is more a matter of seller turn over. I revently sold off 13 guns from my collection simply to reduce the pile to something more manageable and that I was really using. They sold for for less than I had in them. Something I like to think of as gun rent
12 ga vintage guns being in greater supply costs less. This is further aggravated or better said complimented by the current non-tox situation in the UK. I think that the price of good condition and quality 12 ga guns will spur more interests and purchases will return.
SxS guns will remain a niche. The market will not collapse into ridiculous lows in the USA, unless the tom fool silliness of lead prohibition catches on here.
I believe the market for high end high quality guns, Boss, Purdey, high Parker L.C. etc will remain solid.