Then Bitcoin becomes the central clearing house, reserve & store of value chain while other chains are left to operate daily transactions for better efficiency.
People have been proposing this very idea for awhile but assumed the transactions would be handled off-chain by semi-centralized entities. Sidechain removes the need for that.
Miners can mine BTC & the sidechain. I'm not sure where you get the idea they have to choose between the two.
OT servers can do the same thing and give all alts a chance, let it be a market need driven innovation. in my view Blockstream have an advantage, but aside from that changing the SC protocol to exclude MM could happen,