If you choose not to remove the fish and some do survive. The survivors may very well have some ich that they can tolerate but you can't see, most likely in the gills. This means there is ich in your tank and the survivors have developed some level of tolerance. This strain of ich will end it's cycle in about 11 months( it can only complete 34 life cycles). So, if you do not introduce a new strain the ich will be gone in 11 months even if there are fish in your tank to host it. Any new addition during this time,however, will likely be infested.