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.
“Or refuse the deal...”
It may not matter. They can grant you immunity whether you accept a deal or not, and then you are painted into a corner.
“They can’t put you in jail for taking the 5th.”
No, but they can put you in jail for contempt if you try to invoke the 5th when you already have immunity, since testifying can’t incriminate you.