1. Biden's gains over Hillary were largest in red-leaning states like GA, AZ and TX where democrats weren't expecting to need a win , and which only became relevant because democrats did worse than expected in the Midwest. Given Trump's razor thin margins there in 2016 , it would've been pretty hard to hold on to them with even a slight shift toward democrats, which we see even in red states where election rigging would presumably be harder for democrats to pull off. If Democrats were going to engineer a win for Biden, you'd expect them to guarantee wins in the Midwest rather than barely push him over the edge and then "seal the deal" with narrow wins in GA and AZ.
2. Exit polling is consistent with the national vote count being 51 percent Biden, 48 percent Trump.
3. Republicans appear to have held on to the senate, which democrats were favored to take, and also came miraculously close to taking the House. Why not rig those too?