Unless Biden steps down. Which I believe what is planned.  As I said above only 25% of democrats want Biden. 

Is your belief that is planned supported by anything other than raw conjecture?

The closer it gets to the convention, the less that conjecture appears reasonable.  The only reason Biden was the nominee in 2020 was because he was the least objectionable nominee -- the one that all the various factions in the DNC could at least tolerate.  And by default, that means that anyone else who would get the nomination inevitably would piss off powerful groups within the DNC.  Skip over Kamala Harris, and you've pissed off the DNC's most active and loyal demographic block - black women.  Nominate a moderate like Bashear, and you'll piss off the entire progressive wing of the party.  Nominate someone like AOC, and you'll alienate swing voters.  Biden is the nominee because while some of the alternatives generate more positive enthusiasm, they'd also generate more negatives.  And as time grows shorter, you're losing the opportunity to lay the public groundwork to make such a change.

That's leaving aside the issue of Biden first having to surrender the nomination voluntarily before anyone else can be nominated, and there is zero indication he's going to do that.

The democrats are going to pull a switcheroo at the last minute and Trump will not know what hit him.

But you already said this:

Neither Trump or Biden will be nominated. Majority of the country want neither.  New, fresh candidates will be selected at each convention.

Which means that under your theory, somebody other than Trump will be selected at the GOP convention, which comes before the DNC.  So Trump wouldn't be the nominee when the Democrats allegedly spring that surprise.

So if "Trump won't know what hit him", it's probably because he'd be sitting by the pool at Mar-i-Lago sipping Mai-Tai's.

Yellowstone is not a petting zoo.

Oh, you know that's coming.

It's why the vampire needs to be staked (politically speaking) now.  Unfortunately, he's running against a senile leftist who may already have been staked, so the choice isn't appealing either way.

The proverbial race to the bottom.
I can't think of a more absurd 'choice' in all my life.

This has Beirut (1983) and Somalia (1993) written all over it.  Repeating the same damned mistake(s) won't get different results.

If there is an incident involving American casualties before the Election, Biden could lose the Presidency.

Americans are damn sick of wars in the Middle East ... they say it will pay for itself, but it never does ... they say it makes the region safe for democracy, but it never does ... they say we'll get oil out of it, but ungrateful Opec Members, Kuwait and Saudi Arabia, screw us every chance they get ... let the Arab League or the UN deal with this crap.