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.Edited By: Dol Ka on Year 18 Day 268 19:24
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.