$html->input() is almost the same as
$html could be injected only in there is no such variable used in view - in this case user need to use
$this->html->input(). This could also allow overwriting used helper at controller level - you can inject custom helper on
At worst case user could just use
$html = $this->html; at the top of the view to create shortcut with any name - same as import.
I don’t get rest of the points, they don’t look like a problems to me.
This is matter of proper abstraction. Right now in Yii 2 this is not even a goal, because it is impossible due architectural decisions (static calls to helpers - you cannot configure it in any way, so you just continue hardcoding stuff). By using insatiable helper this will become possible and most of bootstrap specific raw HTML could be replaced by constants and or method calls.