National polls are NOT irrelevant. If Biden is winning the national PVI by over 5%, that means he's also won the Electoral College and thus the election.
Biden winning nationally by 8-10% means that he will get a very comfortable victory.
It depends on how votes are distributed.
It's possible (not necessarily likely, but possible) for Biden to win the PVI by 5 and still lose in the swing states if his votes are concentrated in blue states.
How Trump Could Win Re-Election By One Electoral Vote - Electoral Vote Map
In one plausible scenario described by the Cook Political Report‘s David Wasserman, Trump could lose the popular vote by as much as 5 million votes but still manage to eak out a 270 to 268 electoral college win.
Here’s how:
1. Democrats run up their vote margins in California and Texas — two states where the party did very well during the 2018 midterm elections — and take back Pennsylvania and Michigan, states won by Trump in 2016.
2. Trump then narrowly wins the electoral vote by holding Arizona, Ohio, North Carolina, Wisconsin, and Florida.
All I'm saying is Dems need to keep the "down by 15" mentality regardless.