Biden does not have to concede. When Trump is proven the clear winner and selected for a second term by the Electoral College, he will be President for his second term.
Now the left will parade Biden around as the legitimate President. The media will support the fake notion. BLM/Antifa types will be ramping up their violence, probably to include both random and targeted murders.
This may trigger CWII, but I doubt it. If it does happen the legitimate part of our federal government will be on our side as well as the police forces. I don’t think it will last long nation wide. The left coast will hold out until military/milita forces gut them of leftists.
One can only hope and pray for the Electoral College representatives to have ethics, but I wouldn't count on it.
We have already seen coercion and intimidation of election supervisors, and doxing of Trump attnys.
My concern is that the legal challenge takes time, and will not be decided for years.
That would make Trumps legal challenge a "Pyrrhic Victory" : one where you win the battle, .. but loose the war.
Free America will not endure peacefully a prolonged legal battle.