I'm also curious as to what is going to happen when (for example) 50% of nodes have enabled full RBF, 50% of nodes have not enabled full RBF, and I try to replace a transaction which is opted out of RBF. What about if I'm using a hot wallet and not my own node? Will I have to connect to different servers to find one which will relay my replacement transaction? And then presumably some block explorers will show the original transaction while some will show the replacement, and I'll have no idea which one will actually get mined until one of them is mined?
I guess that for a while there may look like a lottery. A replacement transaction of a non-RBF may have a bigger (than 50%) chance to get rejected even if 50% of the nodes are using the new code (unless I misunderstood something). But it should be pretty much okay, we do know that if we want RBF it should be signaled and we do know that a transaction cannot be trusted unless it's mined.
And I do think that after this implementation (when the vast majority will have it in use) the way the mempool works will be more predictable.
Yeah, it's a somewhat bad news for the services that were boasting 0-confirmation deposits. But they can use LN now.