The problem with that reasoning is that there is actually no archeological evidence for it. The idea of the evolution of Buddhist spiritual practices isn't based in anything solid, and in fact the Dzogchen teachings for example completely negate this idea. Anyway, we are far away from DKR and Tsongkapa, probably my fault.
Of course there is archeological evidence for it -- the evidence is in the texts themselves, all kinds of evidence -- from the naming of plants and trees, to locations, etc.
The idea of the evolution of Buddhist spiritual practice in Indian history is based on very solid evidence, inscriptions, statues, etc. There is lot of plastic evidence that tracks to evolution of Mahāyāna into Vajrayāna for example aside from texts.
Irrespective of its historical origins: Mahāyāna is valid on its own terms or it is not. I accept that it is. Bodhicitta is a unique contribution of Mahāyāna to world spiritual traditions. I fully identify as a practitioner of Mahāyāna Dharma. I understand the desire and wish to trace this sūtra or that sūtra back to Shakyamuni Buddha, but I think it is futile.