That's really what makes great studios though. Scrapping projects that don't work, is what keeps quality extremely high. And, it is sustainable to a huge degree.
If you ever own any artistic medium, or do anything else, it's actually part of the process. Sometimes things don't turn out well, and it's better to revitalize a project into something new or make something different. This gives your artists more experience, and in turn makes better games.
Nintendo EAD, Square Enix, Capcom, Naughty Dog, Retro Studios, Rockstar, and even more publishers have literal graveyards of cancelled games(Nintendo in particular has a ton of these). It's how you weed out bad ideas, revitalize new ones, and create quality games. It's perfectly sustainable and it creates better games. And that's why the studios that we know today, are ones with a massive catologue of failed prototypes, cancelled games, and old ideas.
Now, if every game was cancelled, that would be concerning. However, since Blizzard is keeping development with its product that are still very successful. Well.... I mean, they are doing what they doing. Asking a studio to just "push out more games" will just increase crunch, make your games of lower quality, and potentially decrease sales.