That would only work if ALL users decided to move to SCbtc and no user wanted to buy BTC under any circumstances, even with spreads in the thousands of dollars or more. If the tech were that far superior then SCbtc deserves to win out. In reality, there will most likely be debate over which system is better and not all users will convert, resulting in normal arbitration.
People would certainly buy BTC if scBTC were worth more, but only at something less than the scBTC value in order to convert it and make a profit. There would be no reason to convert back
Yes there is reason to convert back. You sell the scBTC at a profit and take the profit and buy more BTC to convert. Rinse and repeat.