Yes, that's true, the game seems to be less popular over time, market is getting smaller over time, you can see it on unusuals and other high-valued items now, nobody wants to buy them, though TF2 won't die anytime soon, people still like trading around for profit, also keys are extremely important for game resellers, as they are being used as payments, though, if TF2 keys will lose it's value on Market, they'll use different items instead, but I don't that'll happen anytime soon, it's still a currency.
The problem with TF2 is that the market's not driven by profit more than ever, most of the market will consist of experienced traders that actually don't care about items self, but only about values, which creates unhealthy market, the item value decrease will be increasing over time, unusuals could eventually go for even few keys, because no one will want to buy them and there will be little to no profile in many segments of market, which used to be easy to trade in.
This is all already happening, it will just get worse over time, some people will end trading and others will just go to DOTA2/CS:GO market, since those ones are on rise (especially DOTA2 market seems to be good alternative, since it's already established). Those are my predictions and I think I'm right, we'll see.