If replication is impossible then how did life manage to get by until now? How do rabbits breed like rabbits if they can not replicate themselves fast enough to overcome failures? I dont really understand the point you are trying to make.
Someone may have covered this, but just in case...
*Perfect* replication is impossible. So, rabbits evolve over time - the rabbit of today is not the same as a rabbit of a million years ago. Go back 40 or 55 million years, and things you'd call a rabbit didn't exist at all. And it is okay if rabbits change over time, as there is nobody who is expecting them to not do so, with a goal that requires they stay exactly the same over a million years....
The issue is that to complete their task of spreading through the galaxy, the self-replicating machines must remain on task, and behave pretty much exactly as they were designed, with no notable alterations. We are probably talking about the most complex machine ever conceived: able to enter a solar system, locate various resources, travel to those resources, gather and refine them (likely pulling them out of gravity wells in the process), and build new machines that can repeat that process tens to hundreds of light years away. Our *entire civilization* cannot do that yet, but we imagine making a single package that can do it.
A small alteration - say, it refines some modestly important element to too low a level of purity - is apt to mean the next generation breaks down before it reaches the next stars, or cannot replicate. Too many cosmic rays through its memory, and it forgets something mission critical. Remember, this thing is getting no operating system updates.
Hell, this thing needs some level of what we might call "intelligence" to operate. We build an intelligence, and send it out alone, not practically capable of communicating with anyone, thinking to itself for decades to centuries. What are the chances that it just goes mad? Or makes decisions in its isolation that doom it's mission? You and I can't stand more than a few hours in sensory deprivation, but you want this thing to handle it for decades?
Someone mentioned the idea that if you can manage this feat, you have probably solved all humanity's problems first - and that seems a reasonable point given all this.