Short answer. No. What matters is that the witnesses were competent when they attested the will. If they were qualified at that moment, their becoming incompetent afterwards does not prevent the will from being allowed. Competency is tested at the time of attestation, not at the testator's death or at probate.
What the law says
If the witnesses attesting the execution of a will are competent at the time of attesting, their becoming subsequently incompetent shall not prevent the allowance of the will.
Civil Code, Article 822 — Later Incompetency Does Not Matter. Read the full provision →
Competency is judged when they signed
Article 822 fixes the moment for testing a witness: if the witnesses attesting the execution of a will are competent at the time of attesting, their becoming subsequently incompetent shall not prevent the allowance of the will. The attestation is a completed act. What the law asks is whether, on the day the witness watched the testator sign and added his own signature, he was qualified to do so. If he was, the will took that step validly, and nothing that happens to the witness later, illness, insanity, loss of capacity, reaches back to unmake it.
Why later events cannot undo it
The rule reflects a simple principle: a valid act, once done, does not become invalid because a participant's circumstances change afterwards. The witness's role was to observe and attest the execution, and that function was discharged in full at the ceremony. Treating a later misfortune as a defect in the will would make every testator's will hostage to the future health of people he chose years earlier and cannot replace once he is gone. So the law protects the will by locking the competency question to the time of attesting, where it can be answered with certainty.
What competent at that time means
For the will to have been properly attested, each witness had to meet the qualifications the law sets for witnesses at the moment of signing, of sound mind, of the required age, able to read and write, and not otherwise disqualified. It is that snapshot the court examines. A witness who was perfectly sound then and lost his reason a decade later was a competent witness for this purpose. The enquiry is not whether he is competent now, but whether he was competent when he put his name to the attestation.
A separate practical wrinkle at probate
Validity is one thing; proving the will is another. A witness who has since become incompetent, or died, may be unable to testify when the will is offered for probate. That does not defeat the will, but it changes how execution is proven, since the proponent must then establish due execution by other admissible means rather than that witness's testimony. So if a witness has lost capacity, the will still stands, but gather the other evidence of proper execution early, because the person who would ordinarily have confirmed it may no longer be able to.