No trade does not mean the price is zero, it simply means there is trade. For example, in a speculative market you would get no trading if everyone agrees on the same price.

Anyway, as I said mining would still happen, and you could potentially buy from miners, but only at better price than what they could get by converting to higher-valued scBTC. There is no arbitrage though, the mining rewards would simply be sought out for their conversion value and miners would compete harder for them, driving up difficulty.