That's nothing but an arbitrary opinion. With my auto-regulation everything would be balanced over time. Let's look at combat for example, as you suggested: Imagine, combat would still be the least rewarding activity, as it is right now more or less. Over time all credit rewards for combat would slowly increase over time, until combat would become the highest rewarding activity in the game (this state would cost some time of course), at which point it slowly will be auto-nerfed again and replaced by the second rewarding activity. Rinse and repeat.
Sigh ... How do you figure out "player activity"? You need to consider that time is the most valuable resource any player has.
Since you mention combat, it takes no more than a single hit of an NPC in a RES and the player only needs to wait for the security forces to shoot down the NPC to collect the bounty. So this "player activity" consists of sitting and waiting for suitable NPCs to show up (which the player has no influence about), using the kill warrant scanner for the 15 (or 30?) seconds it takes to scan the NPC and another second to fire the shot.
Next thing is that players will ask for an increase in the frequency in wich NPCs and security forces show up so that they can make more money with this "player activity".
I basically like your idea. The problem is that when you start to think it through, it doesn't work in too many ways. Is it "player activity" when the player has to sit in his ship for a while waiting to arrive at a REZ with supercruise assist enabled? Or is it "player activity" when the player has disabled the assist? Is it "player activitiy" when the player has to keep holding the button to fire the PWS for a couple hours while waiting in his ship that flys about a ring until he finds an asteroid worth to mine?
I suggest that "activities" that are more time consuming should pay better than activities that don't take much time. Besides that time is the most valuable resource, spending time on doing whatever "player activity" should be rewarded because it means that the player makes some kind of effort and because he is doing gameplay. For example, if you spend a lot of time on a sightseeing mission because you have to do a lot of flying and because your passengers are demanding, that should pay better --- as well as if you have to spend a lot of time on tracking down a mission target.
That also means that if you take a week to shoot down a single NPC and collect a bounty for that, it means that you spent a great deal of time and have done a lot of "player activity" and you should get a lot of money. How much money should you get next week for the same player activity? What if you spent two weeks on it or three?
How do you factor that in?