I really DO get that this might have only ever been intended as a temporary thing.
I mean, if this guy is throwing money at people to go out and scan obelisks, there's probably going to be a point where he decides he's got all the info' from them, right?
You know what would have been a good idea, though?
Put information INTO THE GAME which informs people of what's happening.
Even if you don't want to change the mission briefing, would it REALLY have been impossibly hard to set it up so that Ram Tah sends a "mission critical update" warning people that he's got all the information he needs and as of [date] he's going to reduce the reward?
When you're changing the game in a way that is, apparently, pre-planned and intentional, players should be made aware of the situation via in-game methods rather than having to read a forum which is not, after all, something that all players will do.
Also, as others have said, when you are discouraging players from participating in one of the longest, as well as most lucrative, legitimate elements of the game while STILL retaining the ability for players to stack scanner/skimmer missions (which I seem to recall FDev saying would be stopped) it sends a rather unfortunate message as to how FDev envisage players earning credits in the game.