The Letter makes no sense, Doc can't die

Let's say Marty never wrote the letter, OR Doc never read it (same result).

Now Doc dies. Dang.

However... if we scout the area, what do we see? There's a time-traveling teenager, some probably insignifigant stuff called 'Plutonium' in pellet form that the teenager knows how to load into.. hmm... what else, what else?

There was something else, I am sure of it.. oh yeah, a TIME MACHINE.

You see, 'time travel' makes SO many things irrelevant or redundant, it might be better to stop making stories about it, when you have to force implausibilities or complete stupidities just to make a story happen. There can't be any stakes in a story like this, because the time machine can always undo anything and everything.

Even if Marty does nothing, everything will still be resolved (though there will be paradoxes, but there would anyway).

Let's say Marty goes to 1955, survives the Peabody family's murder attempt, prevents his parents from meeting, and then sleeps in Doc's garage.

Now Marty's parents won't meet so Marty won't be born.

Marty, not being born, can't go back in time to prevent the parents from meeting, so they meet, so.. you get the idea.

The easiest way to resolve everything would be if Doc did absolutely nothing after Marty shows him the time machine.

Doc won't do anything - Marty disappears from existence - in 1980s, Doc won't build the time machine into the DeLorean - no one goes back in time - everything is resolved, no Libyans involved, no Marty in 1955, etc.

So now we come to 1985 where Doc lies dead on the ground, but so do the Libyans. Marty has plenty of plutonium and a functional time machine.

Do I even need to write more?

Of course Marty goes back in time to save Doc, maybe he will take the camcorder to capture evidence of everything so he can show Doc, who will hopefully abandon his hare-brained plan to steal the plutonium.

Of course this would lead to another paradox, but as I mentioned, so does everything else.

In any case, Marty could easily use every single pellet to go back in time to try to save Doc, and I am sure he'd succeed eventually, or at least together with Doc, figure out how to make the time machine work without plutonium (as Doc can easily figure it out in 2015 by himself).

Nothing matters, when you have a time machine, because you can always go back and fix it. It's just like Superman flying so fast he can turn back time - any mistake or death or situation he doesn't like, he can just undo that way, as humorously shown in 'How it Should Have Ended'.

So it doesn't matter if Doc doesn't read the letter, or if Marty doesn't write it, when Marty appears in 1960 to him and tells him what's gonna happen in the future, or in 1975 writes him another letter, or or or... the possibilities are endless. Marty can even take a 10-year break, if he wants to, and still go to one week before the experiment and sabotage Doc's plans in various ways.

Heck, he could tell Doc he wants to show him something, then drive him far away from Hill Valley for the whole night, so the Libyans can't find them.

Marty could secretively sabotage every stage of Doc's time machine plan, making it seem like his invention doesn't work, or depress him so much he abandons the whole plan.

The possibilities are endless, when you have a time machine... so nothing makes sense, there are no stakes, and the letter is useless.