Woah... woah WOAH I wasn't expecting to see this on HN. I've been expecting this for a long time, and if I was valve I would have done something like this a long time ago: release a "final" celebratory content update, port the game to vulkan, and open source the codebase (keeping the item servers and whatnot tied to valve's servers). I don't know if this is the beginning of the end or the end of the beginning of TF2. There have been leaks before but this is huge news.
Beginning of the end? It's been the end for years - they're passing it off to the community, as they should! The team for TF2 is probably very low double digits now and it's almost 18 years old, it's time to outsource the continuing developments.
In the 90’s, iD made Doom, made money off of it for a few years, and then released the source code. They then did the same with Quake. This is part of the reason companies like Valve exist today, as their early games used modified engines from Doom and Quake. Valve is now continuing that 25+ year tradition. People are still making new Doom maps and playing the game. If history is anything to go by, people could be playing TF2 in some form in the 2050’s and beyond.