So the question becomes, “how do you make a hero not worth a damage slot?”
The answer here might very well be that healing is simply OP and needs an across the board nerf.
Mercy 60 hps to 30 hps.
Lucio 12.5 hps to 6 hps and 40 hps to 20 hps
Moira 80 hps to 40 hps and 75 hps to 35 hps
Zen 30 hps to 15 hps
Ana 75 to 35 and 100 to 50
Brigitte 150 to 75 and +75 to +35
Buff their non-healing effects if necessary to keep them viable.
As I’ve been saying, this is not an unsolvable problem. If healing is too valuable, then you reduce the value of healing.