why? you already said consensus is impossible. what migration % of BTC to scBTC will force the implementation? give me a %. and at what % would it be too late?
The consensus required for a total exodus to a sidechain is the same as a hard fork. Moreover, such an implementation will not necessarily require a hard fork.
sigh. then why go thru all the risk and trouble if SC's aren't going to significantly push Bitcoin forward?
They can push Bitcoin forward by enabling features that are not natively available through sidechains that work in synergy with the main one
not worth the risk given the potential for SC to destroy the MC

it's totally different b/c the MM helps bootstrap any SC, with or w/o altcoin.
Any altcoin is able, at this very moment, to be MM with BTC
and yet they're not. but you want to create SC's with altcoins which will enable this on a widespread basis?
Miners decide on whether they merge mine or not. It is not because the model to secure sidechains is merged-mining that subsequently ALL sidechains will be picked up by the miners. They will decide to mine those that are worth their time, exactly like with alts.