All is hyperbole, since the ergonomics on a laptop are terrible, and there will always be a segment for which that's a showstopper, but it's already true for certain segments of the population. Especially among the less privileged where there may not even be a desktop computer available or only at school/library, mobile phones are increasingly where work is done. Writing essays and emails, and filling out important forms, may not be the most ergonomic, but it beats not being able to.

Look at devices with foldable screens and external keyboards like the Asus Zenbook 17. Without getting lost in arguing semantics of if an iPad (mini/pro/regular) sized tablet counts as a mobile telephone, it's clearly not a desktop, and it's easy enough to imagine that desktop computers will go the way of the mainframe.