Because you don't understand a thing about population dynamics, much less genetics. A species with a low population count and a high mutation rate is not likely going to survive long. But that is not the situation with most species. It's a friggin red herring.
Population dynamics ? how in the world are you gonna study something you can't observe
Do you realize how stupid what you're insinuating is. How long do you think two completely different kinds hang around each other once they diverge ? You look in the animal kingdom notice how mostly keep to themselves ?
Where do all the genetic disorders go ? if all organisms were related every friggen genetic disorder would wind up getting passed on.do you think they just disappear ? Don't tell me what I understand and do understand when you can't even offer a rational rebuttal.
Humans alone have 6,000 genetic disorders that are inherited. You can't even come close to matching that number with beneficial mutations. If you understood mutation fixation, you would understand what a problem that is being able to point to more harmful mutations vs beneficial mutations.