How do you imagine preventing an attacker from spending all your coins at once and sending most of their value to fee?  E.g.  you have three 1 BTC outputs which require that the output be a 1 BTC payment to address bc1apple.   A transaction which spends all three at once to a single 1 BTC bc1apple would comply, and yet turning 2 BTC to fees is probably not what you intended to permit.

In my opinion that BIP is essentially focused on a single use case but it kinda pretends to be more generic. The single use case absolutely requires no malleability, and that ends up creating a lot of limitations.  But even without that, additional flexibility is difficult to get right.  I think it would be worth the time to do it right.  The protocol's author disagrees and instead believes he'll be able to ram it down the network's throat really quickly if he keeps it narrowed to his use case.

I hope the network does not deploy that proposal.