I want to say around year 10 or so though I am not entirely sure. When tax planets were a thing they introduced a massive amount of credits to the game; which led to the increase in starting credits for new players.
Hmm.. so maybe it should actually be lowered? I guess the point of starting credits is so you can purchase items at the NPC shop when you spawn..assuming the prices at the shops stay the same then wouldn't there be justification to just keep it the same indefinitely?
Simple answer, the starting credits are for basic shop items and NPC Transport costs. Since the prices for those are set by the game there is no reason for a change in starting credits.
The long answer is that the only way the starting credits will be increased or decreased is if the amount of credits generated in the game increase or decrease by a massive amount in a short time frame. To give you an idea the tax planet era increased the typical faction paycheck from 50,000 - 100,000 to several million, I believe. When Syn mentioned significant inflation it means a very significant amount of credits.
If I remember correctly from my previous characters you start with about 50,000 credits. Enough to purchase three K-4 Security Droids and a basic weapon. The other purpose is to allow a new player to use the NPC Transport system to travel to other Darkness/Quest homeworlds instead of being required to wait weeks for someone from the faction they chose to pick them up before getting to play.
Yes, starting credits will stay the same indefinitely unless most of the credits in the Combine disappear or someone finds a way to generate godly amounts of credits. Not sure I answered your question to your liking though.
The specific reason why the starting credits were increased that last time was because it was possible for a new player to spawn with an amount low enough that they couldn't afford some NPC transport routes. Now they can! It would still be reasonable to reevaluate for an increase in some hypothetical future of extreme inflation, so new players could still potentially buy a cheap fighter or something.