That’s the point; why bother shooting at the Mercy when her team is far easier to kill? 60 health/second (soon to be 50 health/second) isn’t going to sustain a team through focus fire, let alone ultimates.
That’s why her healing averages have remained roughly the same, and for a while, lower than pre-rework; before, she had an ultimate that allowed members of her team to come back into the fight, providing her more opportunities for healing. If used to revive 4+ players, it practically doubled the amount of healing she dealt in that engagement, as the teams effectively fought two fights.
Valkyrie’s healing is spread out and impotent. A decent Mercy would perform about the same in keeping the team alive with or without Valkyrie. The only real bonus Valkyrie provides in regards to healing is mitigation of stray fire, and that isn’t a huge deal in the first place. Focus fire cuts through that rate of healing like butter (especially in high tiers), and the team gets wiped regardless. This time, there isn’t a Resurrect to teammates back from that and bump up Mercy’s healing numbers.