Worth the effort? It's zero effort. They don't have to re-make the game to release it on a different PC launcher. What's the total player count for steam? Peak is irrelevant, it's not an online shooter that needs a huge concurrent player count.

A quick google for steam charts shows shows a 24 hour peak concurrent players of 7,152 players, double your figure. You seem to be looking at the current number of concurrent players, not peak. 7152 x USD$60 is almost half a million dollars, just in people that played at the exact same time in the last 24 hours, for a release that would have taken 1 guy an hour to do.

The game is also more popular on consoles than ever and available on battlenet, where most diablo players on PC buy diablo. Using steam numbers to say diablo 4 is a failure is like the people using Steam numbers to say Starfield is a failure. Those people are eating crow currently seeing as Starfield was the number 1 selling game in the US in its release month and went straight to the 7th best selling game of the year after people called it a "complete bomb".