Rush's future on the airwaves depends on two components:
1) His contract term and personal willingness to continue
2) An Economic Market for his show
Pending #1 is satisfied, it will take large-scale backlash to see Rush's show go. The type of backlash would ultimately need to be in the form of revenue. With 11 advertisers departing the show - this sends a signal... but more likely a small "ping" rather than a siren.
In all actuality, CC's inventory is probably sold-out, so there would be advertisers waiting in the wings. Clear Channel also has a strong national ad sales team, and can target large agencies and corporations for ad buys. Compound that with a presence in most top 150 markets, there are a lot of corporately-controlled outlets.
The real question is with the independents and small market operators. Will local advertisers decide to move their ad dollars to other stations? To other dayparts? In either case, there will be a loss of revenue for that air spot. If there is no replacement revenue, then there is a problem. Most likely, the revenue would be replaced becuase the show still pulls in listeners (albeit it may be slipping in some larger markets with the introduction of the PPM).
My guess, this will have a net zero (or net positive) impact on revenue. It will take a genuine disinterest in his show to impact ratings on a large enough trend to make radio operators decide to change their lineup. With Clear Channel as the syndicator, and with many Clear Channel markets - I don't see a change happening any time soon. Especially if Obama is in the whitehouse until 2016.
The other possible change agent is if an alternative option came about that pulled audience. With Cumulus putting growth into Huckabee there is an option. Maybe Hannity would be a good fit if he were scheduled against Limbaugh ... on an FM station?
Personally, I was a ditto-head back in the early and mid 1990's. Rush was a great entertainer and great Radio person. His show back then was creative, stimulating, and slightly unique in what radio offered. I moved out of the country for 12 years and upon my return and visits over the last 5 years, I have been less interested. I don't know if it's the overall content of the show or my tastes that changed.
Brian