The point is, Bobert, that impeachment is essentially a political process and the commission of a provable crime or even an illicit tryst is not necessary grounds for impeachment or conviction. In the case of Scalia and Roberts, there have been cases where they should have recused themselves on grounds of conflict of interest because of business ties to parties in the cases. (Bush v. Gore was one such case, in Scalia's case.)
There's also a very good case to be made for Roberts and Scalia and maybe Alito to have lied to Congress during their confirmation hearings.   Unlike a jury trial, proof beyond a reasonable doubt is not required.

Normally I would oppose the use of impeachment as a "poltical" tool, but when the Justices themselves are using the courts as political tools, I think it's probably legitimate.   Especially when there's no "recall" procedure and it's a lifetime appointment.