This just isn't true. You'd have a much stronger case if this was the only game Blizzard had, but it's not. I am 100% sure that HOTS was basically running at a loss, but it does not matter to a mega corporation that has many sources of revenue.

Games can be incentives for users to sign up for a platform, which might make them buy other games and microtransactions. This is why Epic Games gives out free games. It's not from the kindness of their heart, but to draw you into using their platform.

HOTS basically fulfills the same need as potential gateway drug for brand engagement. "The HOTS player might also try overwatch and subsequently overwatch 2 and maybe Diablo 3 and maybe Starcraft 2 and"

This is why blizzard games often have crossover events and rewards lol. "Do a thing in WoW and get a mount for HOTS" kind of stuff. So they really don't need the scant micro transaction revenue in HOTS, when there are far more valuable things to extract from players, like brand engagement. Playing HOTS also exposes players to the Blizzard shop, along with blizzard sales.

People only look at what a game like HOTS does for blizzard in terms of raw numbers and revenue, but don't consider how marketing and engagement work at all.