If the sponsor you use have good background then why think the worst 'what if one day...', but of course nothing cannot last forever and I agree with samebb and our ancestors don't depend on one basket.
on the other hand I also think like maxpower. sending your best sponsor the top traffic you sell, offering your surfers an alternative sponsor with a better traffic usage, and of course always be on the look for better converting sponsors. anything can be replaced by a better one. good luck