This is one reason why I prefer Mahayana sutras over Pali suttas, and why I am Mahayana rather than Theravada, even though I have sincere respect for both traditions of Buddhism.
As far as the historicity of the Mahayana sutras:
John W. Pettit, while stating, “Mahayana has not got a strong historical claim for representing the explicit teachings of the historical Buddha”, also argues that the basic concepts of Mahayana do occur in the Pāli Canon and that this suggests that Mahayana is “not simply an accretion of fabricated doctrines” but “has a strong connection with the teachings of Buddha himself”.
Mahayana has not got a strong historical claim for representing the explicit teachings of the historical Buddha; its scriptures evince a gradual development of doctrines over several hundred years. However, the basic concepts of Mahayana, such as the bodhisattva ethic, emptiness (sunyata), and the recognition of a distinction between buddhahood and arhatship as spiritual ideals, are known from the earliest sources available in the Pali canon. This suggests that Mahayana was not simply an accretion of fabricated doctrines, as it is sometimes accused of being, but has a strong connection with the teachings of Buddha himself.
Mahayana sutras - Wikipedia
At least conceptually, Mahayana Buddhism can trace itself to the historical Buddha, much like how a large tree began with a small seed that grew over time.