Until and iff all (or most?) of the BTC has moved out of Bitcoin Core to the superior consensus mechanism side chain. I don't know how likely that is or is not to happen. Just saying.

P.S. If one released an altcoin with such an important improvement, one might think Blockstream could just duplicate it in side chain and suck all the momentum out of the altcoin (thus "stealing work" in the sense of destroying the Inverse Commons by destroying the incentive to innovate in the Bitcoin space in a Tragedy of the Commons). They'd be wrong to assume that is necessarily the outcome. There is an ace up the sleeve.