My opinion is that they that the stand that the only anti-biotic is the penicillin. Other anti-biotics just isn't the original.
The Buddha has given 84,000 Dharma or teachings for diverse beings to suit each types of mind. The later schools is still authentic Buddhist schools for they teach authentic Dharma. We can check it from the results of the practitioners.
To say, only the "original" teachings of the Buddha is the Buddha's teaching is a paradox because:
1) The Buddha's teaching was never recorded directly "live" but were recorded only after his passing. So how sure are we that it is the original. By definition, none is.
2) To equate the Dharma as only coming from the Buddha Shakyamuni is to deny His word itself, as the Buddha taught that he did not invent the teaching but he merely taught the "Truth". There were other Buddhas before him and there will be Buddhas after him. The whole objective of His teaching is to become a Buddha. So if the teaching is authentic, surely there will be others who will be enlightened like Nagarjuna. To deny this is to deny the Buddha "original" teachings itself.
3) If we were to accept the "original" teachings then we must not underestimate the Buddha himself. He is skillful and his methods are still unfolding even now.
So to deny and reject later teachings as not authentic or "original" contradicts the teaching itself.