Sadly we will never know.

Had Biden not stepped aside, the electoral bloodbath would have been even more crimson than it appears to be now.

No, that doesn't matter for the Presidency. It doesn't matter whether you lose by one EV or a hundred. Given how unpopular the Biden Administration was, there might not have been any way for Biden or Harris to have won that race.

The House, and even the Senate, are another story. Pulling Biden relieved frontline House and Senate candidates from having to defend him. From having to defend his capabilities, and from having to defend his choice to stay in the race after the disastrous debate. On the Senate side, the Democrats didn't defend their tough seats (Ohio and Montana - WVa was always gone), and certainly didn't pick up "wish" seats in either Texas or Florida. But they probably lose the Arizona seat if Gallego had to be out there defending Biden.

But where it really matters is the House. Democrats have flipped a few House seats they certainly would have lost if their candidates had to field questions on Biden's qualifications as a candidate - and there's quite a few more that would have been lost to them had Biden remained on the ballot. At best for the GOP, they'll hold the House with a handful more seats than before - enough to retain control, but still probably such a tight majority that they have trouble getting things through the chamber at times. And the Democrats still have a solid hope that the GOP won't pick up seats, or even lose seats - and a faint hope that they might lose enough to give them control.

Biden was an anchor on the ticket. Cutting the rope doesn't mean the boat can win the race, but it won't lose by as much.