Proof of will

        Yes, in my opinion the will is still valid since you wrote that the will is already registered. Execution of the will has already being done in consonance with Section 63(C) of The Indian Succession Act 1925. It does not matter if the the person who executed the registered will died as you only need one witness to prove the authenticity or validity of the will.    
       Since you have written that only one witness is not willing to come forward, you still have another witness in your hand and that will be enough to prove your point (Reference - The Indian Evidence Act 1872). However, there is a catch that the attesting witness should not only prove his attestation but only provide some proof that the other witness ( who is not willing to come forward) had also attested the will. 

In order to have a valid Will, at least two persons need to sign on it as Attesting Witnesses.  Even if both witnesses have died before probate, the Will can still be probated by supportive and colloborative evidence. Though it gets more difficult. For the purpose of probate or letter of Adm, court only requres one attesting witness to come and identify signatures. 

If it is genuine Will, even if both witnesses refuse or become hostile or are dead...it still can be probated. If you want, you can read on Google or Indian Kanoon the case of Syamal Kumar Sen Vs Sanjukta Mukerjee March 2019 where  both witnesses to Will died but the son still got will probated after a ugly legal battle with his two sisters.