Bitcoin block rewards shrink as we know, but soon like 6 years it would be feasible to consider the transaction fee making up a significant part of the block reward. moving those fees to a SC would degrade Bitcoin.
Miners mine both chains. They profit no matter where the transactions take place.
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I mine, i used to MM NameCoin, i dropped it because it wasn't worth the effort and the 2% pool fee i sacrificed on by BTC. nameCoin's fate could happen to Bitcoin, also 14 years form now we may choose to drop bitcoin altogether because its not needed anymore, it worked as a seed for the SC's.
edit:
I agree 100% with your comments on transparency and anonymity but I take problem with your "value leaking" theory. There is no value leaking, it is an essentially an ecosystem sharing value in a synergic manner.
mining must also go through a market evolution, it must stop growing as some point. locking in to PoW MM for ever is not evolution it is manipulation, who knows the bitcoin killer may use proof of Bandwidth or proof of Block Archiving. if you do that with a SC I think Bitcoin would become obsolete as the value will leak into the SC as fast as rewards diminish.