If they want to prosecute him they'll have to state a rationale to get around the pardon. That rationale will be used to prosecute them when they pardon themselves. If they want to weaken the pardon power, they are welcome to. Biden ensured they have to go to maximum lawlessness if they want to do that. They have the power to, no one is disputing that. But in choosing to exercise that power, they will hang a sword over their heads.

Actually it's not that hard. The main legal avenue to invalidate the Biden pardons would be to argue that Biden never actually issued the pardons. He didn't sign them (the autopen did) and given his mental state there's no clear evidence that he was even aware of them. Trump, in contrast, routinely signs executive orders and pardons with news cameras in the Oval Office with him, while chatting with journalists. So it would be hard to argue that Trump was not the one actually issuing them.