People are not gonna buy scBTC on exchanges for the hell of it. To better illustrate :
scBTC-fan wants to get 5 scBTC.
Either he buys your 5 scBTC directly through an exchange or he buys BTC and locks them in the chain with SPV.
No matter the method used only 5 BTC are transferred to the sidechain. No more. Your intermediate sale of scBTC through exchange does not increase the demand. There is not more users willing to buy through exchange than there are willing to use the coin through regular BTC > scBTC mechanism.
I should add to that : your movement of BTC to scBTC is offset by regular scBTC users buying from you instead of acquiring the scBTC through SPVing.
Demand is simply fulfilled through a different channel. Your "speculative attack" does not increase the amount of BTC on the sidechain