I am replying your statement/opinion that they are literary compositions.
I think the point is that: the written records
of the Agamas/Nikayas have all the hallmarks of an oral tradition, whereas the written records
of the Mahayana sutras have all the hallmarks of a literary composition (and very little of that of an oral tradition). Hence we can safely say that, as Greg puts it, "it can be established with reasonable certainty that many Mahayana sutras are literary compositions and not strictly the fruit of oral tradition."
Its quite a logical and reasonable conclusion.
The question is much larger, as I have tried to express. You can also put it this way: Does it mean that there was no Mahayana movement at all, preceding the sutras? Or that the Mahayana sutras are not products from the Mahayana that existed as an oral tradition, as an oral teaching, an oral lineage?
Mahayana has all the hall marks of being a genuine teaching of Buddha Shakyamuni. It has been evidenced by the thousands, tens of thousands, of enlightened masters that the Mahayana has produced. Its proof is strictly spiritual, strictly practical.
It is easy to see from elsewhere in the world, from the oral traditions that have existed on all continents, that the written stories that have been produced from these oral traditions do not differ in any way at all from the style and composition of the Mahayana sutras.