funniest part is people say double spending was possible since 2009.. yet there were no large scale complaints of zero-confirm back then.
people were aware of the risk but dampened their worry by only using it for small amounts

this is because old code knew that transactions relay around the network very quickly. and had policy to ("first see, first keep") reject any second spend attempts, thus mitigating many attack vectors. where the only main successes is to pushtx a second tx directly to a pool that subsequently bypasses standard policy of 'first seen' to add the second tx before the first tx is used in a block

however full RBF bypasses the "first seen. first keep" rule..

its done to force people not to trust using the mainnet for zero-confirms of any amount, just so some capitalists can promote subnetworks as the go to service everyone should use.. yet doing zero-confirms on subnetworks are not to be trust ed either due to MANY ways people can steal funds by their flaws(idiots call features)