IMH(f)O, If you are going to compare TF2 stats to another Valve title, CS:GO is probably a better indication of what may be going-on. I do not thing DOTA is a good comparison - as it (as mentioned by others here) is really drawing a different crowd.
http://www.steamgraph.net/index.php?action=graph&appid=440q10q240q730&from=0 According to those charts, TF2 is still hanging-in there as fars as number of players in concerned. CS:GO has maybe stolen/distracted some players from TF2 and CS variants. TF2 seems to still be holding it's own. No doubt, many old timers have left for good. As much as I may not LIKE it, Valves "strategery" may be working to keep new-gamers happy - at least long enough to be replaced by other newer players. Not sure how long that cycle can be kept going though.
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