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.
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.
I'm sorry but this is a very, very shaky parallel to make.
If Bitcoin is not needed it is only because a better store of value will have appeared. If it is so then let it be, but I don't find it likely since Bitcoin has worked exceptionnaly well so far, benefits from a considerable network effect and has very sound fundamentals.