They could’ve. The problem is that they reworked Mercy and made that disgusting Moth under the illusion that that iteration of Mercy was balanced. You don’t think they did play-testing before they implemented the rework? Nope, they genuinely thought CD Res and a broken ultimate would be balanced and acceptable.
The reason why they took so many patches to fix the issue is because each time they did a volley of nerfs, they thought enough was done until the stats told them otherwise. It was a constant case of expectations being hit with reality until finally they nerfed the one thing that the rework didn’t even change.
All I know is that they’re adamant about her being a DPS.