Republicans likely thought the same thing until the balance shifted their way.I care more about the Supreme Court being completely bogus.
(Also, that’s why criminal trials are done with juries)
Just because you are incredibly biased, and only see things from your side doesn't mean it doesn't matter. The judge allows the evidence that convinces the jury one way or the other.(or doesn't allow it.) It matters a great deal that the law appears unbiased. That is what we are talking about.
The judge is going to have biases one way or the other. This is a matter of appearances and faith. That judge may be able to be a good and unbiased judge most days, or he may not. Appearances matter. It gives Trump grounds to ask he be thrown off the case.
The judge who might replace him could be a very biased judge, but not show it overtly, and then he would be the judge regardless. You already had a biased judge in Trumps favor, in Aileen Cannon. This judge may or may not have been biased, but now we might not ever know.