FULL-RBF Is an attack to Bitcoin's security, it makes every transaction double spendable.
1. Then why isn't that point clearly stated inside the signed message?
2. Should we just blindly follow instructions from an address for which we don't even know the owner? My guess is that most people here are able to make their own mind.
3. Zero-conf transactions are not secure. Confirmed ones are. It's that simple, and has always been. People not taking this into consideration do it at their own risk. (Although FullRBF indeed makes the risk bigger for unconfirmed txs, the previous statement remains true. I can understand that you don't agree with this, but this is not the place for debating it, and you didn't give any substancial argument.)
4. If you need fast and secure payments for small amounts, there's Lightning.
Also, isn't FullRBF disabled by default (opt-in) in Core 0.24? I'm not sure about that one, though.
I'm not necessarily defending FullRBF, I don't really mind. Just showing your lack of discernment.
Anyway, that's off-topic since there's no clear link between the FullRBF controversy and the signed message.