I can't honestly say I understand why people go on about the source engine being long in the tooth - they've evolved the code base ever since the start and add new features with every release; this one will probably be more so as it's been some time since the last major release of a valve game, other than L4D2.
My own engine might have lots of modern features now but it still contains code that's around 15 years old; Because that stuff works and there's no real need to rewrite it.
Still, I'm really looking forward to what Valve have done this time, I have the utmost respect for that team.