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.
Explain to me how you price something that doesn't exist. If
ALL BTC are converted to SCbtc, then that means no more BTC exists. If no more BTC exist then there are no more sellers. Buyers can't buy when there are no sellers and the price is effectively 0.
Bitcoins still exist, they can be unlocked by destroying (or locking) your SC coins, effectively swapping keys with the bitcoins. Their location can still be seen on the blockchain. Really it would drive Bitcoin price up, not down because it is the only secure exchange between SC coins.