Ok, first of all CS is the game that defined or invented tactical shooters and calling it "fast paced" may be a bit exaggerated. Second calling Halo an arena shooter works, but I would not place it there. it is far too slow and the way weapons are handled is not very arena-like.
But all in all the arena shooter is far from dead there are tons of projects around the internet that vary in size but are alive and well off - and then there are Quake Live and UT3 which have quite a big community. Actually to be precise even Quake 1 is still alive an kicking... if you have the skill.
The problem arena shooters have is that they are not ideal for mainstream anymore because they require skill and practice and that won't fit with the current generation of gamers who doesn't want to fiddle around with anything, who just wants to press a button and get some kills. That is why crosshairs now fill the screen, weapons won't require you to aim and movement is extremely limited. So yes, arena shooters are not mainstream anymore but when we take a look at the time they where popular we can see that the same type of gamers that played those shooters back then is still playing them today only that they where a majority back then and are a minority today. Most arena shooter fans have either stuck with their game or spread out over some of the fantastic freeware and open source games out there such as Nexiuz (from which you can get an updated, propriatory version on steam), its open source followup Xonotic, Warsow, red eclipse... and the list goes one and on.
If you want a new fresh arenashooter I suggest you go download Xonotic. It has a small but very healthy and active and growing community and you will often find someone to play with online. It's based on the darkplaces engine (Yes, thats quake 1) and looks quite good. You can google it. If you just want a bigger community you should either check out Quake Live or UT 3. And yes, serious sam 3 BFE also has multiplayer.
The good part about most arena shooters not being mainstream is, that the community is extreme nice and helpful. While playing starcraft you will find a lot of trolling and flaming, you will not encounter anything like that in Quake world, Xonotic or the likes. People here are just fun to play with (although I saw a strange Admin in Nexuiz once you really killed my day, but that was one of countless games and I don't really play Nexuiz).
In My oppinion you cannot take halo as the game that decides weather arenashooter live or die. Halo never really was part of that genre, it was more some strange game somewhere between arcade and arena tending a bit more towards arcade I guess.
So no, just because Halo 4 will be even less of a arena shooter then the other halos that does not mean that arena shooters are dead. It just means that even less trolls will know they exist and that is a good thing!
TL;DR:
Calling arena shooters dead is far fetched and not doing them justice. they just moved from mainstream to underground and that helped a lot to build a better community.