The population can increase in 8 years, but you still have to be voter eligible which takes into account age, citizenship, and registration. The increase for both Biden and Trump comes down to a record-high voter turnout for a Presidential Election. Biden won swing states that went for Trump 4 years ago due to an increase in voter turnaround and yes, some Trump votes went to Biden. Also, Jill Stein and Gary Johnson received a percent or two in some states, which would have made all the difference if that was the case this past election as it was in 2016 for a non-Democrat/Republican candidate.
Also, saying a GOP candidate will not hold the U.S. Presidency again for a generation is a huge stretch. Since 1953 (pre-Eisenhower), no political party has held the Executive branch for 3 consecutive terms.
Depending on how his initial 4 year term goes, how VP Harris positions herself and what her role is under Biden, how the DNC strategizes their campaign, and a whole host of other factors....I just cannot see Biden running for a 2nd term. And if anything like the past 4 years continues for the next 4 to 5 years, I find it hard for VP Harris to ascend to the Democratic Party nominee for the President of the U.S. purely based on her sex and skin color (as shallow and fecked up as that is, I'm just not sure the whole of the DNC nor America is ready for a woman and a woman of color to be President - I could careless if it's a woman as there has been plenty of women political leaders across the world for decades...women can do the job).
And it's not whether or not the DNC or RNC feck up and hurt their own chances, it's more along the lines of having viable candidates who can withstand the bullshit from their own party and rival party, yet be able to connect with enough voters to have a shot at winning.