i speculate that the contention or concern here regards the possibility of complete extinction, including of the Buddha Nature. i will try to further clarify that i believe that extinction is not the ultimate and also that it is debatable whether or not the Buddha Nature becomes extinct.
''They are not shaken by their individuality, and yet their greed, hatred and delusion are not extinct. for non-extinction does not become extinct, nor go to complete extinction. [''Sayings from the Perfection of Wisdom'' p32quote from the Saptasatika Prajnaparamita 27b, Conze.
''Complete extinction'' as above is mentioned in the Prajnaparamita literature as well as the Lotus Sutra [any other quotes in this regard would be appreciated.] it must however be stated that silence leads one to see the external dharmas. one can say that Mind is the ultimate or consciousness (though this does not exist.) and that Mind is seeing, hearing, smelling, tasting, feeling, knowing, perceiving etc, where there is ultimately no experinece nor person to experience.
i will now address the issue of extinction of Buddha Nature as orthodox teaching, though ambiguous:
Zen Master Kumu Facheng, a true spiritual son of Furong Daokai, said to the monks, ''When you realize that there is something beyond the Buddha patriarchs, you will be able to express it. O Zen worthies, tell me, what is that beyond the Buddha patriarchs? He is someone who lacks the six senses and whose seventh consciousness is incomplete. He is the Great Icchantika, [a being] who has no Buddha nature.'' [Quote by Keizan Jokei; Taiso/Great Patriarch of Soto Zen, quoteing Facheng - ''Transmitting The Light'' p 243, Translation; Francis Dojun Cook. 2003.
The above quote is also given by the Koso of Soto, the Eminent Patriarch Ehei Dogen in The Bukkojoji (Going Beyond Buddha) chapter of his Shobogenzo.
There may be some ambiguity over the use of the word ''Buddha Nature''. in Soto this would refer to the ''own nature'', that which allows one to, once seen, perceive emptiness of all things. in Soto this is known to be subject to cessation (extinction). There is however another meaning in use for Buddha nature. This is ''Consciousness''. in the case of consciousness one can say that it is without base and not subject to extinction, ever.
The Encyclopedia of Buddhism says the following of Zhiyi:
Zhiyi characterised the final nature of things as consciousness, which he called Middle-Way Buddha Nature (Zhong-Dao Foxing)[P 741 on the Three Truths, Ed: Keown & Prebish/Pub: Routledge 2010
now, if we say that Buddha nature is Consciousness and not simply ''Own Nature'' it is possible to argue that the Buddha nature can never become extinct. i would from a fundamental position say however that it does not exist and so can already be said to be extinct. though unborn, all beings have consciousness as not having it.
I am inclined to think that The Buddha cannot be excelled in his enlightenment, this is since i believe that he has attained extinction and this is the reason why he is still at work. There is no mind and this no mind is what sees and experiences extinction.
i hope Queequeg that this clears things up.
with best wishes and respect, Tom.