JS evolution disrupts the clean-break rationale.
Languages can change both surface and semantics. The open question I see is: can JS become the VM (its source the "bytecode") to host many languages and several approaches (none memory unsafe; no shared-memory threads).
We have lots of evidence for JS becoming the VM already, along with nay-sayers who want a clean break.
I'm going with evidence over clean-break assertions until we hit a wall that is either inherent in the language or emergent in the standards body and the market. Again, this is why I am concerned about Google playing fair.