I wouldn't say TF2 itself was rapidly declining in popularity at all, Doomie. If it's declining at all it's barely noticeable.
The dozen pub servers I have on my friends list are still full most nights of the week. The competitive scene is just coming off the back of a number of highly successful leagues worldwide, and correct me if I'm wrong Lefty but I'm pretty sure ETF2L Season 10 was the biggest in terms of number of players... ever?
Highlander is gaining more and more of a solid and respected place in the competitive scene too, which can only prove to be a good thing!
P.S. Oh, and don't forget PubComp (http://pubcomp.com/
). It is going to be completed soon... I hope...