But as I said, they are ultimately bound by the laws of nature. Evolution can only happen so quickly; they can only evolve their genome so much in a given period of time.

What you're suggesting is they'll evolve to look like human cells. But that doesn't make sense. They would need to evolve that way in a single generation. Something that's "almost like" a human cell would just be wiped out.

Just consider how long it took for the last common ancestor (of humans and bacteria) to evolve into a single celled organism.