I'm making assumptions but I don't think it's asinine to assume that they would rather run their service with a constant steady income from a subscription model rather than a one time purchase over the period of half a decade. It would have been one thing if they released paid expansions to the base game every few years but they never did that.

It also doesn't matter if the company as a whole is profitable if Overwatch itself isn't. They aren't Kirkland with the food court hotdogs or rotisserie chickens, as in trying to use Overwatch to pull people into their other games even though it's a net negative for them to continue to support.