Never did them much favors either. And you have to remember, that doesn't appeal to everyone in the Republican party. That sort of tactic is better suited towards the Democrats, because of who they have to appease within their own party. It doesn't work for the Republicans, who prefer to be more conservative in how they act.
Does he deserve it? Maybe.
Is it a good idea? No.
You have to look at it from Mitch's point of view. If he pushes for impeachment (which won't likely make it pass the House, let alone the Senate), the focus and agitation will shift from the Republicans to the Democrats. Right now, a lot of Democrats are hammering the Biden Administration for what it has done. But it's not enough for them to risk the Republicans regaining power. So if there's an impeachment, there's a good chance enough congressmen will flip. And the Democrats are going to go in damage control mode.
Instead, let the media continue to hammer the Biden Admin without the fear of impeachment. Let the Democrats own this. That way when election comes around next year, Mitch and his allies can do a hard press on what happened in Afghanistan. Going for the jugular now just muddies the water. I know it isn't what you want to hear and maybe because you serve, you feel more strongly about this, but that's the situation.
Because I expect the next two-three years, with the pandemic raging, is not going to go too well for Biden and the Democrats. Let them own the damage.