Why can't we just be in insert mode constantly and use Ctrl+something for all the commands? Also use Shift+arrows to select text?
If you have any issues with RSI, then vim is by far the best editor (especially the GUI mode version).
The command mode allows you to formulate in your mind what you want to do, and accomplish it in the minimum possible key strokes. This is important because your brain won't wear out like your joints and tendons can. The command mode usually allows you to specify repeat counts to avoid repeatedly mashing keys. It lets you make common text selections such as words, nested brackets, statements, etc with a couple of key strokes. It also allows you to create macros on the fly and repeat previous commands further cut down on repetition.
I find that reaching for the arrow keys puts my wrists at an awkward angle which induces more stress. Having to pound on them repeatedly while holding down various combinations of control keys makes things worse. (To address a common complaint with vim, I've added a mapping of Alt+F to ESC, so I don't have to reach up for that key if I don't want to.)
I probably spend more time in command mode than insert mode when I'm using vim to write code. That says to me that keeping an editor always stuck in insert mode may not be the best idea.