Or he may have feared that the hypocrites might cast doubts...
Or he may have feared that the hypocrites might cast doubts about the authenticity of such writing due to its being done under the influence of sickness, thus becoming a cause of dissension; therefore, he said: "The Book of Allah suffices us," supporting the verse of the Almighty: "We have not left aught (without explaining it) in the Book (Qur'an, 6:38)" and also "Today have I completed your religion for you (Qur'an, 5:4)," out of his own concern, peace be upon him, for this nation against straying after Allah had completed His religion for it and complemented His blessing unto it.
Such was their answer. His saying "... you shall never stray" indicates determination and a positive attitude. The endeavour to bring about security against straying, whenever possible and without any doubt, is a must. His disappointment with them and his telling them to leave him since they did not carry out his order is another proof that the matter was simply a response to a consultation.
So, if you say that had it been a must, the Prophet, peace be upon him, would not have repealed it simply because they disobeyed him, just like he did not stop preaching due to the opposition of the unbelievers..., if you say all this, then we would say that the case is so had the order been carried out, for it indicates that the writing of that matter was not obligatory on the Prophet, peace be upon him.
This of course does not imply that they should not have brought him the sheet and the ink-pot when he ordered them to, explaining to them that its benefits would include security for them against straying and a source of continuous guidance. The main point is that those receiving his order should have obeyed it, especially when the benefit was for the one receiving the order, and it is the reason for the statement, not for its enforcement.
Yet it is also possible that it was obligatory on him, too, and such an obligation was removed due to their insubordination and their saying that he was speaking in delirium, for the fate of such writing was then reduced to dissension, as you yourself have wisely stated.
It is also possible that some people may say that ‘Umar, may Allah be pleased with him, did not take the meaning of the Prophet's statement to imply that such writing would result in protecting each and every member of the nation from straying, so much so that none of them at all would be misguided. Rather, he understood "... you shall never stray" to mean "...