There are definitely enough people playing on Mac's to justify continued support of the client. Plus developers are still making macOS ports. The Mac App Store isn't very popular for games and most gamers tend to prefer Steam still. So that's just money on the table.

Valve hasn't updated their own games but that's a whole a different calculation. Wouldn't be surprised if they never update those to run on Apple Silicon or whatever else they change to in 10 years.