This could be a ploy to attempt to get sombra into the meta, though I think it's a stretch. She probably has the easiest time getting behind enemy lines undetected, shouting aside, and her EMP can destroy a cloistered set up, including forcing Bastion out of Sentry mode. Also, regarding your points about overpowered abilities pre nerf:

Zenyatta's Discord wasn't a problem at 50% because he was one of the easiest heroes to take out and negate the effect, resulting in balance. With his survivability buff, discord needed to be toned down. In addition, the 50% buff wasn't something new on the PTR, it was there from the beginning

Ana's Grenade only had one buff, that being the 1 meter increased radius

Roadhog is made to punish out of position players with massive damage, they just reverted the broken hook that punished players who were in position

Nano Boost is as powerful as many other ultimates, and they even nerfed the speed boost it provided. Ana was initially thought as underpowered so they saw no reason to nerf anything to help her viability. It's an example of an unconventional support ability, buffing your allies so they can get eliminations more easily

Soldier badly needed the damage buff to compete with other DPS characters, though I do agree he needs a slight nerf to his damage

D.Va's survivability was partially based around the tank meta, since it was so easy to heal all tanks up to full. Yes she's durable, but requires focus like any other high priority target. It's when she's under focus and she still can't die that there's a problem.

Sombra is not meant to be a hero who goes in and gets a bunch of elims, she's mainly there to disrupt. Blizzard has stated that the way Sombra's made to be played is not how most have been playing her