Well there will be a time to come when something will be done to have a permanent show host in that 12NOONPM-3PM time slot. In time it could be Mark Steyn or someone who will take those hours.
I think you are right. Eventually there will be needed a regular host. I don't think the show will survive as many might hope as a continuous Rush retrospective. Perhaps on either Monday or Friday they could run a complete 3 hour Rush show from the past. Yes, this is very sad for all his regular listeners and of course his family.
The best person to take over his show, at least in the short term, is President Trump. Give him 3 hours a day with a massive audience and you’ll hear liberal heads exploding left and further left!