If it is said:
“Could it not be that he was present on all those occasions, only no one could see him?”

the reply is:
The basic rule rejects such a far-fetched supposition, for it would restrict texts of general import on the basis of nothing more than conjecture. Besides, what would have impelled him to stay hidden when appearing openly would have increased his reward, raised his station, and made his miraculous sign all the more evident? Moreover, had he actually remained alive after the Prophet ﷺ, the most excellent course for him would have been to transmit the Prophet’s ḥadiths and the Qur’anic verses; to denounce forged reports, inverted narrations, innovated opinions, and partisan whims; to fight alongside the Muslims in their campaigns; to attend their gatherings and congregational prayers; to benefit them and ward off from them the harm of others; to guide scholars and judges; and to formulate proofs and rulings. All of this is unquestionably superior to the claim that he merely roams through cities and traverses deserts and distant lands, meeting worshippers whose inner states are unknown to most people and acting as their spokesman or overseer. Once this is made clear, no one will hesitate about it after proper explanation, and Allah guides whom He wills to a straight path.

Among the evidences for this is what is authentically reported in the two Ṣaḥīḥs and elsewhere from ʿAbdullāh ibn ʿUmar: the Messenger of Allah ﷺ prayed the ʿIshāʾ one night and then said,

“Do you realize the import of this night of yours? By the time one hundred years have passed, not one of those who are now upon the face of the earth will remain.”(1)

In another wording: “Not even a blinking eye.” Ibn ʿUmar said, “People became confused about the Messenger of Allah’s statement, whereas he only intended the passing away of his generation.”

Imām Aḥmad said: ʿAbd al-Raz̤z̤āq narrated to us; Maʿmar informed us; from al-Zuhrī, who said: Sālim ibn ʿAbdullāh and Abū Bakr ibn Sulaymān ibn Abī Ḥathmah told me that ʿAbdullāh ibn ʿUmar said: The Messenger of Allah ﷺ prayed the ʿIshāʾ one night near the end of his life, and when he had given the salām he stood and said,

“Do you see this night of yours? At the completion of a hundred years from now, not one of those on the surface of the earth will remain.”
This was also recorded by al-Bukhārī(5) and Muslim(6) through al-Zuhrī’s chain.

Imām Aḥmad likewise said: Muḥammad ibn Abī ʿAdī narrated to us, from Sulaymān al-Taymī, from Abū Naḍrah, from Jābir ibn ʿAbdullāh, who said: The Messenger of Allah ﷺ said shortly before his death—about a month earlier—

“There is no soul that breathes—or, none of you whose soul is breathing today—that will have a hundred years pass over it while it is still alive at that time.”