Other way round: they are immortal because they don't have transcription and replication errors.

Maybe, but immortality also tends to come with things like extraordinary capabilities for self repair and by extension disease resistance, so maybe they get some sort of error correcting code in their DNA replication to go with it.

Whale cancer resistance

Imagine this: you’re immortal, not just in the sense of “not aging,” but in the sense that you cannot die. No matter what happens, disease, trauma, fire, bullets, you persist.

Now introduce cancer into the equation. You don’t get a merciful end. You don’t get release. The cells in your body still mutate, still misfire, still turn against you. Tumors grow. They spread. They choke your organs, twist your bones, invade your brain.

In a mortal body, this means death, an end to the suffering. In your body? It means endless malfunction. Organs clogged with growths but never failing enough to kill you. Skin splitting under lesions that never close. Your lungs filled with tumors, every breath a shallow gasp, but the suffocation never comes. Your brain riddled with metastases until you’re half-aware, locked inside a prison of decaying flesh that never stops decaying.

And it doesn’t stop there. Given infinite time, you don’t get one cancer. You get all of them. Over centuries, every tissue eventually betrays you. Your body becomes a battlefield of competing malignancies, tumors devouring tumors, a grotesque ecosystem of your own flesh.

You live forever, yes. But you might pray for death long before eternity runs out.

Wrong way around.

They're immortals because they've left their cancer riddled bodies behind to become made of living metal.

Or I've been reading too much 40K.

I dunno, I guess we'd have to ask that crayfish that cloned itself a few million times and will likely never die.

All immortals eventually become Deadpool.

Maybe immortals are just lobsters without a shell ¯\_(ツ)_/¯

I highly doubt immortals maintain the same body parts due to this. Kinda like a car. Parts have to eventually be replaced.