I will point out mechanism of gain (i.e. how one goes about earning something) is different from rate of gain (i.e. 1 story point per 5-10 minutes of play time). If you could buy 1 story point for 100 million credits, you'd probably say the rate of gain has gotten slower.
If the issue is rate of gain, then it's probably more effective to address that directly rather than shifting incentives for activities around which then get calculated to late game incomes and late game rates. It'd be weird if story point credit costs scaled with levels, meaning you'd be incentivized to stay low level, make lots of credits (i.e. trading), buy your ships, buy your s-mods, then level up to 15, at which point the credit costs for story points are higher.
If you don't do that, you won't be able to afford anywhere near the same number of story points you have now (because it's calibrated to late game income), or alternatively, you remove any meaning of story points at late game (as it is calibrated to early game income). Although the later might be the desire in such a change.
So one really needs to answer the questions of:
What amount of effort should produce what gains or changes in your fleet (or colony empire) in the late game? How should that differ from early and mid game? How long would a typical playthrough take to reach said stages of the game?
Should I be able to completely retool a 240 DP fleet, going from a low tech Onslaught/Legion/Erradicator/Enforcer fleet I'd been using the entire time to a new high tech Radiant/Hyperion wolf pack, in 5 minutes of personal time at end game? I'd hate for it be along the lines of waiting an hour AFK while letting colony passive income roll in. At that point, I'd rather just see at level 15 you achieve infinite story points rather than incentivize AFK play. It's also why I like the idea of a super simulator (either in campaign or probably better as stand alone mission). It lets you try out things in an experimental way quickly without risk or investment.
At the end of the day, ideal rate of gain depends on what you think you should be able to do with X amount of effort/time in the late game.