Perhaps that's the problem with this idea. Having a system that makes you perform the same activities in the game if you want to stay on top is going to be seen as something that prevents some players from trying other things. There will be some players who will want to stay at the top of the leaderboard and they'll probably grind themselves to boredom. Not how I would play, but I'm sure the devs would be aware that's what some would do.Well, you can gain exploration rank by only scanning previously discovered bodies (road to riches) and trade by hauling biowaste (I presume...)
But sure, for combat it will be very slow. The chart posted above shows that you always got some points for destroying ships at some levels below you. FD just made a change that all kills gave a very small amount of points towards rank. As you say, it would probably be more fun (and certainly much quicker) to do it by picking better fights.
The ranks in ED are simply in game motivators or goals, nothing more, and it is I am sure intended that they be achievable by everybody. The idea of them being an indication of anything other than time spent performing an activity isn't really feasible, certainly they are not indicative of skill. That said, acquiring skills is also a valid in-game goal which I'm sure many players work (or play) towards for their own satisfaction (and would certainly make the attaining of the in game goals quicker).
From what I can gather from the OP, his idea is simply the same but on a shorter cycle. If you want to be 'Elite' in combat, then you had better spend a lot of time just blowing up ships to stay in the top x% of players blowing up ships, otherwise you're not worthy of the rank, and it will be taken away from you. Not convinced that would be popular, especially as not everyone has the same amount of time that they are able to commit to playing the game.
It's probably one of those ideas that would need to have been implemented from the start. Thing is, back then it was seen as bad enough that there was no offline mode among some players. It would probably still be too much multiplayer 'interference' in the game for some solo players.