My opinion (which has no influence on whether Arduino would add such a feature) is that Upload Using Programmer is so rarely used that it's not worth making the UI more complex by adding a button for it.
If you're using the mouse anyway, it's no problem to do
Sketch > Upload Using Programmer one handed, though that is a little bit slower. You can also do Ctrl + Shift + U, but I'll admit that particular keyboard shortcut is tricky to do one handed. In the case of someone who is disabled, they would likely already be using some software that allows them to create custom keyboard shortcuts.
It is possible to configure the board definition so that Upload acts as Upload Using Programmer when uploading to that board. This is done by not defining an upload.protocol property in boards.txt for the board. Some hardware packages where there is no bootloader have Upload configured so it always acts this way. An example of that is
ATTinyCore, which always does Upload Using Programmer for the boards that don't have (Optiboot) in their name.
Other hardware packages add a custom Tools menu that allows you to choose whether you are uploading via a bootloader or via a a programmer. An example of that is
MiniCore, which has a
Tools > Bootloader > No bootloader option.