You would need to have a "unregister" process as well, which would help. Also when someone goes online it would check the code. So we're talking about that half with no net access somehow going back to the days of disc swapping, doubtful that'll become really popular again, of course unless these people voluntarily keep the console offline (which is possibly the case in a fair number), but in the end we're down to about 25% of the market potentially doing it, then we have to consider how many would actually do it and you're not talking the massive loses you're pointing to.
It's not the perfect solution right now but the next generation will have an even greater online focus making the solution even more viable. It's really not that far fetched once you consider the entire circle.