I’m not convinced of that. Every sniper in game does a better job at it than Mei and can one shot her, while she requires two shots + wind up time.
Additionally, it help allow her to apply pressure to teams that are pushing in or getting ready to. An example would be Rein shields, having no fall off makes her damage just slightly more effective when working with her team to force Rein to recharge or break shields.
I mean, that could have also worked as well. There’s a couple ways to get around to accomplishing the same or similar goals. I’m arguing against the idea the OP had that she had amazing damage. She doesn’t, she consistently on average has the lowest of all damage dealers.