I’m not the person you were responding to, but found your question interesting.
I re-read most of the Wikipedia article on Markan Priority. Imo These parts of the article sum up the argument nicely.
While Marcan priority easily sees Matthew and Luke building upon Mark by adding new material, Marcan posteriority must explain some surprising omissions. Mark has no infancy narrative nor any version of the Lord’s Prayer, for example.
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Nor does Mark have more than a handful of unique pericopes. This is expected under Marcan priority, where Matthew has reused nearly everything he found in Mark, but if Mark was written last, it is harder to explain why so little new material was added.
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There are very few passages in Mark with no parallel in either Matthew or Luke, which makes them all the more significant […] If Mark is drawn from Matthew and Luke, it is hard to see why so little material would be added, if anything were going to added at all, and the choice of additions is also rather strange. On the other hand, if Mark was written first, it is easier to see why Matthew and Luke would omit these passages.