The mRNA vaccine mostly stays near the site of injection: your arm. This is why your arm mostly aches, and the rest of your body not so much. A bunch of cells there will die, because your immune system much prefers an innocent cell dead than a infected cell continuing to multiply. But you have plenty of cells. Really quite a lot. So it is entirely worth it.

We can try to optimize how much protein cells will make from the mRNA. The goal is "as much as possible, for as long as possible", but mRNA is not meant to be long lived. And foreign mRNA trying to make its way into cells would be destroyed very quickly without trickery (like the nanoparticle and the pseudouridine).

But either way, medicine works just the same (a surprising amount of the time!) in the cases where the precise details and mechanisms haven't been elucidated yet. If it works empirically, the details are just details.