There can be microscopic cancer cells that are not visible on scans. They usually develop over many years even decades and they learn how to survive in the body. And until science learns how to kill them all, up to the last cancer cell, especially the invasive type, you need to listen to your doc. If he goes for some clinical trial there is a risk that the cancer spreads beyond the bladder. It's interesting that even the best treatments have different effects on different patients. There are always some patients that don't respond to the drugs. I read that checkpoint inhibitors showed a great response for bladder cancer (as well as for lung cancer and Hodgkin's lymphoma), but it's very risky to rely on it. What if this does not work for a particular patient. Usually they take only those patients that have no other alternatives.