Of course they won’t drop a good chunk of their existing subscribers overnight; once the initial work is done, maintenance shouldn’t take a ton of resources, especially considering it’s still a cash cow. Likewise, they are sort of maintaining SC2 with these tiny balance patches, and it’s still working on macOS, but it’s clearly the last echo of the old company.
Blizzard _today_ takes to maintain World of Warcraft's macOS build. The same Blizzard is releasing all new titles as Windows-only.
This difference between WoW and there rest is what puzzles you parent.