"The Biden Pardon immunizes everyone from future prosecution"

And there's a pretty huge precedent for that; the preemptive pardon of Nixon.

You're making an argument for why its use is defensible. I find it not unconvincing, especially since it's pretty much just Analects 13:18. But Trump can use the Biden Pardon (shorthand for broad large-period pre-emptive pardon) too, and he's pioneered the use of the Trump Pardon (shorthand for plausibly deniable pay-to-pardon). The combination of the two pardon techniques signals the end of Rule of Law for sufficiently well-connected individuals in the US. Plausibly Jeffrey Epstein was just caught a decade early. He wouldn't be in trouble today.