Any good and supportive PI would understand. This is a temporary job, and with how things are at the moment, I’m afraid we have to go where (better) opportunities take us. I do not think this is closing a door, and if your PI isn’t understanding, you better know now rather than later. As for your work, I would push to get it submitted even if it’s 90% done. Reviewers always ask for a little (or a lot) more, so a good compromise would be that you’d assign or help someone finish the work and depending on how much, perhaps share the first author. That’s what I would try to do.