I think you missed the point. The fact that such situations are so rare SUPPORTS common descent - the odds of a retrovirus causing a DNA are very rare so it doesn't happen very often at all.
That is why the fact that humans and apes sharing common DNA fossils is such strong evidence.
"are very rare"
Mind-bogglingly rare. Assuming that it "probably" happenes" as the poster said.
"That is why the fact that humans and apes sharing common DNA fossils is such strong evidence."
That's where you lose me