While Overwatch may not have achieved it's lofty ambitions, it's still extremely well-regarded [0], had 50 million players (which is quite impressive given that it's not free-to-play) and has grossed over $1 billion. It arguably brought the modern hero shooter genre to mainstream popularity and caused a flood of similar games to follow it. You're right, nowadays there are more popular games in that genre, but at the time it was quite a novel concept.
Since you said 10 years since the last 'decent' game from Blizzard, I assume you referred to Diablo 3. I wonder, by what metric is Diablo 3 a decent game and Overwatch isn't?
[0] https://en.wikipedia.org/wiki/List_of_video_games_considered...