I highly doubt half the subscriber base will be leaving with Howard. I am a huge fan of his and, although I don't plan on having a subscription without him on their airwaves, I only have an internet subscription. If my Sirius radio were never stolen from my car I guarantee you I would keep my sub because the service has much more value in a car than on a computer IMO.
The only thing that will have a large effect on the subscriber base is if Howard moves to another subscription based service for his show. Then fans will have to decide which subscription they want to pay, and I doubt many will opt to pay for two services. However, I am sure that Mel Karmazin is aware of this and is strongly negotiating for Howard to not move to another subscription based service.