I don’t think it should be two accounts because that will break interoperability between Threads and Instagram. But having said that I agree that you should be able to disable/opt-out Threads with your IG account.
When you first sign up into Threads it will ask you if you want to follow everyone you follow on Instagram. This will allow you to follow people that are already on Threads and it will automatically follow the people that will sign up for Threads later. But you can’t tag people that haven’t signed up for Threads yet. So it’s definitely opt-in but it has a discoverability mechanic built in to make it easier for you to find people + potentially follow people that may sign up in the future.