That's just a distorted view you have there, McDee.
Back in 03, LfS was quite simple. So new features were easy to add, quick to test and art, due to the low polycount was quickly done.
Over the years though, LfS grew very complex. New features, even if they are minor, interact with the existing code, thus it needs more time implementing, testing and fixing things. Additionally, graphics took a big step forward, which also take much longer to produce to the desired level of detail.
So it is really a no-brainer that the "added stuff per month" ratio sinks. That doesn't mean that the developement became slower. For all we know, Scawen could hack in more lines of code per hour than ever before.