Mining now offers a variety of approaches, which is great. I see no need for "balance". I do the activities I want to do, not necessarily the activities which are easiest, pay the most credits, preferred by other people or intended by developers.
Consider what approach is the game leading you by the hand to do then? Motherlode --> Motherlode --> Motherlode?
Consider at the moment how the game would even allow you to undertake mining surface and sub-surface deposits to make a reasonable income?
Consider when was the last time you used an abrasion blaster or sub-surface missile (on a non-motherlode asterds) to obtain reasonable amounts of what you were after?
Consider if a more balanced, varied and interesting mechanic might lead you to mine asteroids rich in surface deposits you're interested in, with a nice further reward being ones with some valuable sub-surface deposits, and then sometimes you coming across a motherlode one?
As far as I can see it is exactly as you describe in that sentence. That players ignore anything other than motherlodes is down to the player choice. Other options are available and do not need to be removed.
Try it... Go to a hotspot, and locate asteroids (eg: using the PWA) not specifically looking for motherlodes. You'll simply find asteroid after asteroid after you'll ignore....
ie: The game rewards your efforts with failure after failure after failure to find anything of interest/worth... So why do it?
That's what I'm experiencing at least with the current mechanics...
As I've said countless times in this thread, when was the last time you used an abrasion blaster or sub-surface missile on a non-motherlode asteroid. Why is this?

Consider if a more balanced, varied and interesting mechanic might lead you to mine asteroids rich in surface deposits you're interested in, with a nice further reward being ones with some valuable sub-surface deposits, and then sometimes you coming across a motherlode one?