Note: The following is a strictly "what if"-type of topic. If your only commentary will be "It's never going to happen", please move along because you are (1) only stating the obvious, and (2) showing off your limited intelligence by only stating the obvious.
What if both Joe Biden and Donald Trump were replaced on their respective tickets before the general election?
Point 1: While Bernie Sanders was campaigning for the 2016 Democratic nomination, the Democratic National Committee (DNC) very pointedly expressed that they were a "private organization", and were under no obligation to choose Sanders even if he received a higher delegate count at the convention in July than the DNC-favored Hillary Clinton. In the same statement, the DNC representative also stated that, as a "private organization", they are under no obligation to follow even their own rules.
Point 2: Both Republican Donald Trump and Democrat Joe Biden are (at the time of this writing) presumptive nominees. They will not become their party's actual nominee until they accept the nomination at their respective conventions later this summer.
Point 3: There is a rumored "general malaise" within the DNC about Joe Biden's ability to campaign for the general election in November. This, in turn, leads to discussions on who could possibly be selected by the DNC as the Vice-Presidential candidate to bolster the campaign, and thereby bring in a Democratic Party victory in the general election. Logic suggests that if, for some reason before the DNC convention in August, Biden was found to be unable to maintain what is certain to be a rigorous campaign schedule until November, the DNC could simply replace Biden with the VP pick and then choose another VP. (Remember, they are a "private organization" and are under no obligation to follow even their own rules.)
Point 4: A similarly rumored "general malaise" within the RNC about Donald Trump has been given legs by the recent RNC decision (as reported by Newsweek magazine online) to hold a reduced convention in North Carolina in defiance of the president's wishes. There have also been reports of a legal "cease and desist" demand of CNN by the White House to stop reporting the results of an internally-conducted poll that shows Trump losing to Biden. Given that the path was opened by the DNC that they were not required to follow their own rules, and that the RNC is also a "private organization", logic suggests that the RNC could, if they chose to do so, also replace Trump on their party's ticket with someone that they believe would win.
Following the path of points 3 and 4, it would seem to me that the Republicans have an advantage because their convention happens two weeks after that of the Democrats. If the DNC chooses a new candidate just before (or at) their convention, the RNC will have two weeks to brainstorm about selecting a nominee who can defeat the newly announced Democrat.
So, the next logical question would be "who would they choose"? I won't name names, but there are some features that are worth considering.
Let's look at the Democrats since they have the first convention. Hushed rumors have run rough-shod over the party base (with neither confirmation nor denial by the DNC - they're playing it very close to the vest) about a woman being selected for Biden's VP running mate. A few names have been openly speculated, mainly those who ran primary campaigns, and several candidates have even been reported to be jockeying for the best position. While speculation barks like a dog at the mailman over who might be named as the VP candidate, there is another angle from which to view this. Joe Biden is well-liked by both the party base and the party elders, so anyone named as a VP and/or potential replacement candidate would almost be required to be even more liked by the party elders (and the party base) than Biden. That narrows down the field considerably.
The Republicans have the more difficult task. While it's true that they will have about two weeks to consider who would be best suited to run against any newly named Democrat, it's far more likely that if they do reach a point where they would start to think about a replacement for Trump, by that time rumors would have already been whispered and picked up by the media. There were only a few weak challenges to Trump's nomination in the primaries earlier this year, so it's unlikely that any one of those people would be selected by the RNC (sorry, Bill). Just as it would be with the DNC, anyone that the RNC would seek would need to be both well-known and well-liked by the Republican base to even have a chance. Most of the candidates who competed against Trump during the primaries of 2016 have kept a very low profile since that time, but that's probably where the RNC will need to go fishing if they are to find anyone who might be willing to hitch up their wagon this late in the game.
As I said, none of this is supposed to be taken as "real"; it's just a "what if" game that I play in my head. If you wanted to know what I do for fun, this is it. Oh, and PlayStation.