As far as I can see, no one is diminishing Mahayana.... only saying that buddha-nature clearly isn't buddha-vacana.
So for you to call it one of the Buddha's teachings is erroneous from a Theravada point of view.... rather, it's from "discourses that are literary works — the works of poets, elegant in sound, elegant in rhetoric, the work of outsiders, words of disciples"
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"The uprooting of identity is seen by the noble ones as pleasurable; but this contradicts what the whole world sees." (Snp 3.12)
"To argue with a person who has renounced the use of reason is like administering medicine to the dead" - Thomas Paine