Anti-transaction replay in a hardfork

In an email exchange on January 24th, 2017 at 8:03 PM, Johnson Lau pointed out that there is a difference between an opt-out system and an opt-in system. He suggests that the requirement for an opt-in system would be self-defeating as it would force unmodified yet compatible systems to change in order to support the new network. This requirement is unlikely to be included in new network designs. Instead, Johnson Lau proposes that an opt-out system would make more sense for everyone involved.