thus having control over the immutability of the EOS blockchain.
ECAF doesn't have the power to control users' accounts, but BPs have. ECAF can only propose transactions which are finally made by BPs.
This doesn't have anything to do with "breaking immutability", because it's still preserved. All transactions are recorded in the blockchain and nothing is removed. The blockchain is immutable.
It's about multisig: BPs have the right to create any kind of transactions from any account, so basically all accounts in EOS blockchain are multisig accounts which are controlled by the account owner and BPs. If ECAF is removed from the constitution, users have more to fear because ECAF is supposed to an independent institution in this multisig scheme. Without ECAF, there is nobody to tell BPs what kind of multisig transactions they can make with the users' accounts.
RE: Proposal for an incremental Constitution and Dapp layer governance on EOS