I agree, even 60% is unrealistic in XIV's current playerbase. I'm just saying, there were people who struggled with Storm's Crown EX because it did more frequent raidwides than usual, and that's nowhere near even 60% healing uptime. There are healers who can get 100% DPS uptime in those fights and solely heal with oGCDs. Actual healing uptime is when you will naturally be supposed to use Medica II in the fight and refresh it in such a case. Healers will have a correct and proper rotation to juggle your oGCDs healing and keep track of those CDs to the second to actually handle the healing, or the party wipes, all while spending most of their time GCD healing + handling the fight mechanics. GCD healing is used primarily to make up for the lack of healing that oGCD healing cannot cover, not for healing mistakes. For people who want 80% healer uptime, Healing a mistake with a Cure II will be a luxury, and you're naturally going to have to forcibly heal through different methods to make up for the lack of potency (ex: substituting 1 of the 3 raptures preplanned in Plenary with a Cure III instead because Medica II is required to be ticking and you lose more upfront potency by overwriting it). That's literally what people mean when they want actual "healer uptime" -- where you can't afford to be spending GCDs to DPS because your core gameplay is to heal large amounts of unavoidable damage primarily. Those oGCDs are all not optional like it is in current content where you can reduce a big portion of your DPS uptime for more GCD healing.
In hindsight, the existing concept behind Slyphies is not particularly bad per se. All Healers could be slyphies when they do new content because they are more focused on learning the fight and responding to mechanics / keeping the party alive rather than DPSing, but DPS healers are basically the ones who gotten comfortable with seeing & responding to the damage and can start DPSing after. The problem is that the players that are referred to as "Slyphies" never graduate from that mindset to do efficient healing to find space to DPS, and that's why they're generally struggling when they start actually have to heal in large amounts where each healing skill has to be used to its fullest. They never developed a cognitive load to be placed in a situation for it as they're used to the current ease of difficulty. Increasing the requirements directly screws them over because they're not like the DPS healers who do efficient healing so they have spare cognitive load for DPS rotation.
Now people expect them to actually be efficient at healing AND be able to keep healer uptime to the point there's very little room for error? No matter how much I could want healer uptime, it's just unrealistic in the current design space for healers.