biosynths VS Cyborgs, futurism VS Science Fiction

nanites will permanently bond with human cells and empower them to do all sorts of things.

Its not just injecting new DNA- which BTW wouldn't work. Its actually over seeing every chemical and biological process on the nano scale.

The "limitations" of DNA which you seem to be thinking of thus are just limitations at certain tech levels.

DNA could be for instance sealed into bucky tubes, and whole cells could be sealed into protective membranes.

"> Well, so how do such civilizations overcome the limitations of DNA? DNA after all has to conduct its functions using the water-filled protein-driven medium of cells, which have temperature, gravity, nutritional and radiation tolerances much narrower than similar beings made of metal, computer chips and electric power generation? "

All of those abilities can be grafted into live cells, which is over all a better option than starting from scratch for a variety of reasons.

The main issue is cell repair, which is now done by the body in assorted ways but which could be taken on by nanites.

Aside from that the next issue is cell protection, which again can be done by giving cells new protective barriers.

"Injecting DNA through the medium of nanites"

No, the DNA would be programmed into Nanites, which would arrange life building blocks according to the new patterns.

It wouldn't be injected, thats the long hard way.

"still counts on the growth and maintenance functions of human bodies to make the changes, human bodies which must continue to maintain that fragile state called "life" throughout the process of modification."

Yes, thats a list of problems.

"It's like trying to write a masterpiece of literature using only 1/4 of the alphabet, and requiring iambic pentameter throughout."

no, its like adding modern building materials to the mix where people are building mud huts. You have a larger and more dynamic batch of new materials, new machines, new chemistries, and new architectures which
you can deal with, quite the reverse of your idea there.