Once you accept the fact that the meme of the dying desktop and mobile only comes from people who are "creative" by answering mails/reacting on social media and dragging things from A to B, you will quickly understand that the desktop on which most of the grunt work gets still done will still be there for a long time.
If you ignore the lag from the CPU or memory constraints, the interface is just not conducive for working. Who wants to edit text documents or spreadsheets, used a shell, or edit code on a 5 inch screen? Who wants to use a soft keyboard for any non-trivial amount of work? It's not that it can't be done; there are much more attractive options readily available.
There there's the argument about connecting mobile devices to monitors, using adapters/bluetooth for peripheral devices, and thin client setups. If that's the case, why don't you grab a cheap laptop? Mobile is gimping you at that point.