Just my two cents:
Literacy in the past wasn't at all what it is today, today in a majority of the world's societies, its a serious handicap, an obstacle to leading a fully functioning life, & certainly an obstacle to an intellectualy satisfying life. In the past, the Avarage Joe & certainly the Avarage Jane got on perfectly well while being illiterate. And even with the lack of literacy, Muhammad didn't live in a cave amongst aborigines, he lived in a society with poets, poetesses & a significantly vibrant cultural life & some Christians' & Jews', from where its possible for any illiterate to pick up the requisite skills to construct a Holy Book. There's some bits of the Quran which are sublime & have considerable literary & ethical value, apart from the threats & intimidations to unbelievers on practically page, but nothing to indicate that a Divine Being wrote it, even nothing to indicate that a Shakespeare like literary genius penned it.
Allah must have realised that his work isn't up to snuff, for after claiming that His surahs are the best & challenging others to write an equally good surah, He warned unbelievers that they can't & if He doesn't like the surahs they've written to challenge it, He'll burn them in Hell!
There were many very intelligent people in the past who functioned in the highest stratas of society, & functioned brilliantly while remaining illiterate-the Mughal Emperor Akbar was an illiterate, & a paragon of good kingship. Although his illiteracy was due to his determination not to study as a child & it certainly wasn't a practice to let rulers be illiterate.
Today when we need clothes we go to weavers & tailors & we don't consider it a handicap not to be able to spin clothes ourselves,in the past when people wanted something penned down, they went to scribes-& weren't in the least bothered by their inability to read & write.