Brook mentioned before that the only thing that could destroy him is to completely crush his bones. If that thing doesn't happen, will he be immortal?
If he really is immortal (or will have a longer life compared to other SHs), I hope we don't see the day when all of the strawhats dies (because they are mortals) except him. That would be so heartbreaking especially to Brook. I think I won't be able to handle it for him to sing, laugh and live alone again.