It’s very difficult to justify the additional effort of implementing a platform that serves exclusively the playerbase with a ~3% market share

And yet there are many games that have a native Mac port and no native Linux port, such as the recently released Ball Pit: https://store.steampowered.com/app/2062430/BALL_x_PIT/

How is it to justify that a platform with an even smaller install base gets the native port? Two ports, actually, because ARM and Intel are both natively supported. Why aren't Mac users expected to use Whisky to play Windows games but Linux users are expected to rely on Proton's battery munching API translation? Apple is even worse in breaking compatibility, so game developer cannot even expect their Mac games to still run in five years.

The problem isn't "the playerbase with a ~3% market share" because 3% is still millions upon millions users in absolute numbers given the massive PC install base. According to https://www.theverge.com/pc-gaming/618709/steam-deck-3-year-anniversary-handheld-gaming-shipments-idc there were 6 million Steam Decks sold last February and Linux is still rising in Steam's Hardware Survey. According to a bit of googling, Steam hat 1.5% Linux users that month, a third of that using SteamOS.

I'm too lazy right now to extrapolate even a rough ballpark of the overall Linux user base on Steam but even if we assume that a big number of Steam Deck buyers doesn't use their device, I don't think a user base north of 10 million is too far fetched.

So the problem isn't the 3% number, it's the developer's / publisher's attitude to expect that Proton just works without any QA and that Mac users are somehow valuable while the Linux peasants are not.