The ich can and will survive in your Main Tank for a specific amount of time. I am not up on the EXACT life-cycle of Ich, but let's say it is 4-5 weeks. Any parasite needs a HOST to survive. It cannot live ANYWHERE where it is not supported by a host organism. In this case, ICH needs a fish. It cannot be hosted by a shrimp, or coral, or something else. This is why you leave the tank entirely fishless for x amount of time. The parasite can survive for a set amount of time without a HOST. That is it's normal life cycle. So by leaving the tank fishless, with nothing for the ich to attach to for it's life cycle, it cannot survive. So it will survive for a certain amount of time, then once they have all died, you can return the fish. If you put your fish back one day too early, it may have defeated the entire purpose of the quarantine. Like I said, my 4-5 week is just an example, and I am NOT entirely sure of the time for Ich.
Also, although copper can be great, hyposalinity will work too(very low salinity content), but read up on either treatment, as they are both very controlled. Like VERY, VERY specific amounts of copper, AND very specific levels for the salinity. If either are off, the ich may not die.