Not sure I understand what you said.
"Healing as less as possible can be done by any healer, because they have the choice to not healing at all."
Healing as little as possible means you keep your party alive and none of your healing is wasted. If someone dies, you haven't "healed as little as possible". Minimizing healing has the direct consequence of causing very little overhealing. Meaning your MP is not wasted. Your GCDs are not wasted.
There is a minimum amount of healing in any content: The unavoidable damage that would kill a player. Those healing requirements can go up the more player makes mistakes. Devs have designed 8 man content to require 2 healers in most of the cases. The fact that some healers decide to heal as less as possible and just DPS while their partner focuses on healing and still clear the content is NOT an argument against healer DPS, if anything, it makes evident that there is not enough incoming damage to warrant a second healer.
If anything having healers have so much downtime means the tuning of incomming damage is at fault. It's a design problem from the devs: The fault is not with healers who focus on DPS.
If there is an expectation for healers to DPS it's because game design accomodates such expectation- In any MMO, wether healing dowtime is prevalent or not, if you can chose between a Healer that can keep the party alive, and a Healer that can keep the party alive AND deal damage or contrubute to clear the content in any other way (CC, Buffs, Debuffs) you will always chose the one that can do both.
Throwing "But some healers let their party die to dps" at this argument doesn't work. Because those healers ARE NOT keeping their party alive. We agree that the bare minimum for the role of a healer is to heal their team.
You will clear a fight wether your party members stay at 50% HP during all the fight or 100% HP. Heck, the only HP that matters is the last one. When a healer player becomes more experienced, they can push the limits of their job in their content to make sure they maximize their contribution. As there is nothing else to maximize their contribution to the team beyond DPS that's what they focus on.