…It would’ve directly competed with an active game that Valve are still monetising, using the same assets, on an engine made by Valve themselves. Compare to Black Mesa: a remake of a very old game on a new engine using altered assets; and even then, Valve asked for a cut.
The outrage is undeserved. Doubly so because the devs themselves said they were stopping production anyways due to technical difficulties
The point of the fan campaign isn't specifically about the dmca, but moreso the apparent apathy of Valve towards the state if the game, with the item servers diving down for about two days and bots flooding official server still not having anything done to practically combat them.