So you are saying that the 'dsigner' went to the trouble of inserting each of those 'thousands' ERVs rather than just placing it in a common ancestor? And that is the simplest explanation?
Placing the sequence in thousands of different organisms is easier than placing it one organism and using natural speciation. Hmmm. Interesting.
No, the designer originally designed viruses as DNA-sharing mechanisms. They do that quite well.
It was the fall into sin that resulted in some of them becoming malevolent.
We have already seen that ERVs help moderate insulin activity in reproduction. More discoveries are sure to come as understanding grows in this area.
That's a design feature, not a 'lucky' accident of evolution.