There are still several thousands players of Halo on Steam. When the game launched it made it up to #3 on the active list. Now the game is under well down the list.
I have been grindly through the campaign on Legendary and it has been slow progress. I have not played Reach previously so I am new to the game.
Many of the current player population are using the multiplayer components. Halo is designed originally as a first person shooter, the multi-player is simply added after the success of Quake Arena etc.
I have tested the game with my GTX 1060 and my RX 480 and both cards can play the game at 4K fine. I do note that the mainstream 1920×1080 is still 2/3 of the Steam group. The cost of higher quality monitors can exceed a high-end video card.
The next component, Halo: Combat Evolved should be delivered sooner or later. The original CD was very popular both on the Xbox and later on the PC.