Would this impact the defensibility of Bitcoin? One could make the case that, because it is now easier to cancel a transaction before it is confirmed, then it is harder to defend your Bitcoin.

This argument doesn’t hold: Everybody will agree that unconfirmed Bitcoin aren’t your bitcoins to begin with, so there isn’t anything to defend. Only way to own Bitcoin is to get it mined into a block. The reason is that there is no consensus on what the list of unconfirmed transactions is.

Another case can be made: Does full RBF impact the portability of Bitcoin? John would maintain that it makes it harder for people to spend their Bitcoin and it makes it harder for the merchant to accept Bitcoin. This argument on the surface has some merits but fails to convince me. [...] If we were to accept John’s argument, almost anything could be considered as impacting portability and thus be unethical: For example with the Taproot soft fork, we got a new address type. Suddenly, some people sending Bitcoin with old wallets wouldn’t be able to send to some other people with Taproot wallets. Should those people assert that introducing a new address type is unethical because it makes sending bitcoin harder for them? Same test can be used: If 100% of people supported taproot addresses, or if 0% of people supported taproot addresses, in both case Bitcoin would still be a perfectly fine money! Thus, introducing a new address type is ethical.