They've sold it continuously over the last 20+ years, and continue to do so. I bought a copy this past year to try out Project Diablo 2[A], my second or third purchase of it, personally. They remastered it and sold that too, while selling D2, side by side. I also bought that to play with a friend. Quality sells.

It should be an example to all companies that a game made with passion can produce money for a long time. Diablo 2 was also one of the games that launched Blizzard into the stratosphere of popularity (which can be turned into wealth directly and indirectly). Awards. Prestige. The obsession with cash is out of control.

It might actually be a fat tail, if that's a thing, given how long it's been selling.[B] In fact, blizzard has probably made more money on me from D2, than D3, which I bought once, and I have not purchased D4 for a number of reasons.

[A] https://www.projectdiablo2.com/ - a lot of fun

[B] https://en.wikipedia.org/wiki/Diablo_II#Sales

[Edit]

Wait, what exactly is a waste? Not charging a rent? aka a subscription?