For the time being, I personally do not miss Resta because it at least pushes the player into playing more nicely (and from what I have seen every class is equally capable of dealing damage and avoiding damage) so this isn't exactly a situation like the original Phantasy Star Online 2 where a class like the Ranger and Force had to avoid attacks and enemies in a very different way compared to the Hunter or even the Successor classes.
One of the obstacles I feel that gets in the way of "difficulty" in Phantasy Star Online 2 is currently that in the context of Ultra Hard everything ends up being "get one-shotted or take very big hits that you can fix with a Resta/Megiverse/Mate" until you pick up the Etoile where suddenly then you're just tanky enough to take a few hits and negate the need to really play "smarter," defensively, or evasively when you can face-tank everything. From what I have seen in New Genesis, the enemies have been retooled and balanced in such a way that taking a hit seems to matter in that you don't have "infinite access" to something like a Resta that will facilitate sloppy play. All the classes existing have such a range they can attack from and also the means avoid taking damage that it definitely seems like a heavy-handed push for players to get better, at least for the time being.
In other games like Dragon's Dogma Online the healing classes have the ability to heal health but the maximum health restored gets reduced the further players take damage so you will eventually no longer get to 100% restoration until you use an item manually. This gets circumvented with a sort of power-creep far in the future where a very hefty grind eventually granted the pure support class the ability to restore health beyond the maximum damage threshold.
In closing, I do not exactly see this as so problematic at the moment largely because the Resta Signs are so easily obtainable through the field and during combat that I haven't seen any players really "struggling" to deal with the content because it was always relatively easy to heal and revive others without too much difficulty.