The definition of humans isn't based on how many organic limbs you have.
It's more of a question wherever you're still a human at thought.

This appears to be a slippery slope argument. In this specific case, there is a very prominent barrier in the form of the fact that people will hesitate to replace more parts than is necessary due to ingrained ideas of humanityYou don't seem to explain why this could happen. You seem to be underestimating the amount of money and expertise it would take to do this. Organ transplants have existed for ages; illegal and involuntary organ transplants have only been occurring a relatively tiny amount of time.

Humanity is going to be a heavily subjective concept; what is your definition of it, for clarity?

So could a lot of things, like someone setting fire to your house. However, the benefits of the technology far outweigh the possible repercussions (here comparing the invention of fire to various transhuman technologies).

¬¬

Fine, well then, I have no problem with Transhumanism, robotic parts, half-humans half-machines, and all that things.

In the case this could generate war-machines, I'll back down on what I said.