I think that the approach to this potential problem should be similar to that of a potential server hardware failure. Keep up to date backups and ensure that you have control over your domain registration. While your site may be down a few hours during a transition to another company or server, you still have control of your site.

Regarding the use of larger companies, I partially agree with idcdc. However, using a smaller company that provides added value in terms of support, etc. is many times beneficial. The key to avoiding potential problems is to ensure that you have a current backup and the company uses a larger/stable provider.