As in most team based games that revolve around multiple classes working with each other, playing as a Tank or Support Hero in Overwatch can be rather ... unsatisfying. There's glory to be had in being able to provide an impenetrable wall for your teammates as Reinhardt or keeping your teammates alive through obscene amounts of damage as Ana to be sure, but like a modern day Pavlovian experiment, the audiovisual indicators you get for kills in virtually every game encourage people to, well, get kills and win, and the easiest way to get those red skulls and dinging noises to pop up in Overwatch is to play as a DPS-oriented Hero, which would partially explain their overwhelming popularity.
That being said, there are some Heroes in Overwatch who are uniquely equipped to fill a specific, slightly more passive, role, and this maxim is practically personified by Mercy. While Mercy has a sidearm to retaliate against attackers, she has always been the go-to Hero for people who are looking for a relatively consistent and powerful healer and little else. Her latest ability reworks do little to change her identity as a primary healer, but they do make her slightly less one dimensional in that while she will no longer be able to revive an entire team at once (Resurrect will be a basic ability with a 30 second cooldown), her Ultimate ability will soon make her significantly more versatile by giving her the ability to fly, deal more damage, perform a degree of AOE healing, and revive teammates on a slightly more frequent basis.
Similarly, D.Va is a Tank who can occasionally snag a few kills here and there, but her most prolific ability is her Defense Matrix, which is the only basic ability in Overwatch that can completely negate some of the game's most devastating Ultimate abilities. As of now, it lasts for some four seconds after which it goes on a 10 second cooldown, but that is a small price to pay for potentially being able to prevent Heroes like Zarya, Mei, or McCree from annihilating a team. If D.Va's proposed changes go through unscathed, however, her ability to prevent incoming projectiles from impacting on your teammates' faces will be severely compromised, as Defense Matrix will last for two seconds. In exchange, she will be able to fire a barrage of direct-fire rockets that can theoretically deal heavy damage to 200-health Heroes, or at least force people to not stand in one place and eat rockets.
In a vacuum, Mercy and D.Va's proposed changes are actually pretty healthy for Overwatch. Highly skilled Mercy players will simply be able to survive longer and contribute more to a team than their lesser skilled brethren while D.Va players might enjoy the ability to score more consistent kills as well as being able to deal damage at a range greater than five feet. Furthermore, the proposed changes shouldn't really change your team's targeting priorities that much when playing against such Heroes, as Mercy is still a high priority target and D.Va must be dismounted unless you want her to eat a Graviton Surge or High Noon, so it's not like games are going to devolve into a mad free for all anytime soon.
It should be asked who truly benefits from such changes given Overwatch's more recent updates, though. With Roadhog's nerf and the introduction of Doomfist, the pendulum of balance seems to be slowly swaying in favor of DPS Heroes and those who prefer to exclusively play as them; after all, there were few things that discouraged overextension quicker than a hook and a shotgun blast to the face, and Doomfist is essentially a one-shot overextension machine. If D.Va's Defense Matrix were to last for only two seconds, then there's no longer going to be much that stands between her massive weak spot and a wrecked MEKA, to say nothing of how there would be less of an incentive to keep track of whether or not D.Va is in position to intercept a potential team-wiping Ultimate. Similarly, despite Mercy's newfound survivability while using her Ultimate, that she can no longer revive two or three people at once puts less pressure on DPS Heroes to actually do their job and hunt down healers before they save the team from getting wiped off a point.
Intentionally or otherwise, such proposed changes have also had an effect on how people perceive such Heroes. A quick Google search will reveal as much as there are frankly a worrying amount of websites, YouTube videos, and forum posts that concentrate almost entirely upon Mercy's potential damage during her Ultimate ability (which is apparently getting toned down for final release anyways) and not upon the fact that she could potentially heal up to 300 health per second if the entire team is tethered to her healing beam. And D.Va's rockets are, well, rockets, so there is an obvious appeal there. There is a potential danger here in that while such changes could encourage formerly DPS-only players to try out other roles in Overwatch, it is also entirely possible that such players will get a bit overzealous in their rush to deal damage and get kills, much to the detriment of the rest of the team, not to mention how two utility-centric Heroes got tweaks that are decidedly not entirely defensive in nature. It is mostly doubtless that playing as a Support Hero in Overwatch will generally be more satisfying in the near future as they get more defensive/offensive options, but care should be taken to ensure that the game doesn't turn into a contest of which team has more raw damage.