This is something you'd learn in your first year at law school. If, as Trump's complaint alleges, Dominion did destroy evidence then the evidence is immediately assumed to support the opposing party's allegations.
Biden's defense would now have to prove there was no fraud as if Trump's team had certain evidence of there being such.
What you're saying is: Biden has to prove something DID NOT happen.
Do you know how impossible it is to prove a negative? I'll give you a hint: in this particular case it's literally impossible.
It's really easy in this case, Biden (not literally Biden) has to prove that each ballot counted was a legal ballot. The evidence that can conclusively prove it is there, but they're destroying it.