“I won’t shed a single tear for Cosby but were I a criminal I would seriously think twice about taking a deal which forces me to testify in exchange for immunity after seeing this go down.”
Criminals usually don’t have much of a choice. If they are offered immunity, they may lose ability to invoke the 5th amendment to avoid testifying. Then, if they don’t answer questions, they can be tossed in jail for contempt until they do answer.
but the truth is that many complex cases (like RICO stuff) relies on some people to take the deals offered in exchange for their testimony and without it they cannot get to the big fish. Take that off the table by prosecuting people anyway and once word gets around it will be stone silent on the stand. Sure you'll put away some grunts you caught red handed but you won't unravel the network.