People don’t stay exactly the same over a period of 10 years, especially when exploring a time period of change, as happens in your twenties and thirties.
As Johnny Galecki noted in Glamour, the characters had to mature and evolve over time.
We may miss the days when the guys had trouble attracting the affections of the ladies on the show, but if we had seen them do the same thing through the past 10 seasons we likely would have given up on the CBS sitcom long ago.
While some of the funniest material happened back in the days when characters like Leonard and Sheldon were far more into fun debates and geeky conversations than in attracting and keeping girls around, the show has definitely turned into something with more talking points for the average fans.