Its not always just about tabindex. For example it might be about understanding that there are multiple ways to fill out a form that makes sense. Then hiding/showing pieces as needed and/or providing hotkeys to jump from field 1 to field 5 because the user doesn't want to fill out 2,3,4 because they are optional. Its about keystroke optimization. Sure they can press tab 3 times, or they can just press ctrl-5 (or whatever) to get there.

If you watch people use the sabre command line interface (the one from the 1970s?), you can see some of what i'm talking about when people are just filling out the forms with the submission line, its less using the GUI and more just knowing some sequence of keystrokes that results in an action being taken.

AKA its possible to do both, without having the user wear out the tab key, or grabbing the mouse all the time.