The trouble is that healers have more HPS than what the DPS of the opposing team, unless they can coordinate their burst. Healers also have too much sustain and resources behind the HPS they put out.

There are many ways they can rebalance this:

- Reduce DPS / increase HP and drastically reduce HPS and put more strategy on sustain (makes poking a valid strategy but might make ranged even more powerful than melee as a side effect)
- Put diminishing returns on healing in some form. Either a stacking debuff on the healed target (something that lasts only 3-5 seconds) or decrease healing potency when casters MP gets low. This allows opposing teams to dive on someone who gets out of position.
- Give DPS more tools (like I dunno, unrestricted communication) to perform burst and coordinate burst

There are many examples of PvP and how healing works in them, and you can look beyond MMOs. Look at MOBAs like Dota or League, or even Overwatch. Not in any of those games can you have the entire team wailing on someone and the target being kept alive by nothing more than raw HPS. It takes coordination, peeling, and other support abilities like absorption shields or short invulnerability shields. I think a big part of the problem is that healing is just dumbed down to keeping line of sight and dumping large heals on whoever is being targeted.