There is a serious lack of understanding here. The transponder is used for identification of an aircraft ... not detection. The plane is still visible to radar. Taking out the transponder does nothing to the radar system ...
Can the radar tell the size of the plane? Or could a large plane say they’re a commuter plane on VFR (just make sure they don’t use the callsign for a plane that the particular FAA sector is already monitoring) and then squawk as directed - and go wherever they want to go without raising suspicions at all?