Your domain is like your car. It can be crashed in a moment and you’ll have to get a new one. Your membership site is based on your domain (I am assuming) so if you lose the domain, you lose your members.
Your list is your ‘real’ estate. If your host shuts you down and keeps your domain name, you can rebuild with your customers. If you don’t have a list, your business dies with your domain and you are back to square one.
Sean’s suggestion has real value. I had a site hacked a while back and I would have like to have been able to secure or move my domain name which I could not do. There is a real story here but this is not the place to share it.