~ I'll have no idea which one will actually get mined until one of them is mined?
Isn't that the same now? If you use RBF, there are 2 valid signed transactions out there, and it's up to the miner to choose which one to include. The one with the highest fee is most likely to get confirmed, but there are no guarantees.
I think that after the change this should be
more predictable (but still not 100%). Since a new tx is treated as RBF (maybe if it has bigger fee though?) it should have "the upper hand" when picked by miners.
Of course, since one's mempool is different from other's, the chance for the initial tx get confirmed first, even if the second one has the bigger fee, cannot be reduced to 0.
Isn't it the same now with all the tx that have RBF flag on?