This has never been true. TF2 is still actively developed by Valve, the community just supplies art assets, it's Valve that implements them. Does anyone say Mario Kart 8 was mostly Monolith made, because they were outsourced as asset creation? No, that'd be ridiculous. Valve still needs to implement, test and then maintain anything they add to the game. For weapons they need to balance, test far more extensively, and maintain for a lot longer with more serious consequences on the game if they mess up.
This is also a weird thing to say now, considering they've released a few major updates within the last few months adding new features and modes, and they're building a massive competitive oriented update to be released in a few months. Something that will have a huge impact on TF2, and something the community is not developing at all.
I've also seen contributors mention that the royalties on their assets is actually at or above the usual royalties offered for out sourced assets. A single model in a video game is not going to give the artist 10,000 dollars within the first few months, and then a steady income of a few hundred after that. And yet that's about the average for cosmetics (or was a few years ago, not sure where it is these days).