can a on-screen keyboard be used as a complete substitute for a physical keyboard in terms of speed and accuracy etc. is there any downside?
Depends on the person. I remember when people were trying to sell me software back in the 1990's and I would say "Why do I have to type all that when I should be able to click a dropdown?" Today I operate a keyboard a gazillion times faster than a mouse. What works for you? There's your answer.
If I have my way I do away with my mouse. Keyboard traversals are so much faster for me. But Microsoft (Office suite) has taken away a lot of the keyboard shortcuts I used to use often, to my dismay. The best combination for me (on a laptop) is a keyboard and a trackball. Very few models provide trackballs.