Why is it that only I need to show that my scenario is more likely? Why don't you need to show that individual infections followed by moves to fixation for *thousands* of ERVs is 'more likely'?
OK, I think this makes it clear.
In your scenario, if I'm understanding you correctly, there are thousands upon thousands more fixation events:
Under the accepted scientific scenario, an ERV got inserted into the genome of a chimp-human-gorilla ancestor once, then it was inherited by the daughter species.
Under yours, the insertion has to happen three times, in the exact same spot in the genome, and get fixed three different times. Sounds like three times more work. It also sounds less probable; why can't one of the fixations fail?
Now consider an ERV found in all apes but not in monkeys. Using normal biology, the process is exactly the same as for the chimp-human-gorilla case, just earlier. Under your hypothesis, the same virus has to infect the germ cell line of every species of both Asian and African apes in exactly the same location, then become fixed in each lineage. Lots more work, much more chance for failure, again.
Because common descent is *assumed* 'a priori' and everything is shoved into that paradigm? Obviously.
I repeat, the ERV data is evidence for common descent; as I showed above, common descent accounts for the tree-like distribution in a simple and elegant manner. No ad-hoc assumptions about the various species of simians having their DNA created with the tree pattern so the retroviruses all infect the same part of the genome. No blasphemous assumptions that God is really Loki in disguise, putting unused reverse transcriptase genes in mammals just to fool us. No improbable assumptions that the ERVs in all the various species got fixed, or that all species in a clade (everything descended from some ancestor) got infected, and only them.
Continued research will destroy this scenario just like Haeckel's embryos. It just takes a while.
Be careful what you wish for, it might come true.
There were two things with Haeckel and embryology that are no longer part of science. First, his drawings were not accurate. Not a big deal; they were replaced by photos decades ago, and are now only of historical interest.
Second, his Biogenetic Theory, that, say, a developing mammal actually is fish, then an amphibian, then a reptile, and so forth, is wrong. This has been known since before 1900, and, again is only of historical interest.
However, ontogeny really does recapitulate phylogeny, and provides yet more evidence of common descent. There are a lot of phenomena that simply make no sense otherwise. Examples include
the fetal teeth of anteaters and baleen whales which are reabsorbed and never used;
the teeth of platypuses, which are always below the gum line and are never used;
the way a whale's blowhole starts off as an embryonic nose that moves back;
the egg teeth that some newborn marsupials have but never use;
the way our ear bones form in the jaw and migrate to the ear;
the recurrent laryngeal nerve;
and so on and so on.
Some More of God's Greatest Mistakes is an amusing discussion of these and related facts of natural history.
