WHM is arguably in the best shape it has been in terms of lily usage, but taking away stuff from WHM consistently has already reduced its gameplay over the years. If it was at one point a 4 and then it dropped to a 2, just because it's in its best state as a 5, doesn't mean it's still good when the bar is up to a 10.
Also, What GCD rotation are you talking about? All WHM has is Glare and Dia. Assuming you're including Afflatus spells, you also have to remember that Afflatus spells fall under conditional usage - You use Afflatus spells to access Misery, but Afflatus spells are ultimately healing, and healing is part of a binary condition where you simply use it when people need healing or to avoid overcapping. There's virtually no thought to be had in it in most cases because of how much excessive healing you have in the first place. It doesn't matter if WHM suddenly has "4 GCD spells" in its rotation, if you're healing when you don't need to, it doesn't feel good. The GCD rotation doesn't feel rewarding and lacks engagement.
SGE barely has a different GCD rotation than WHM in all honesty. They have Dosis and Eukrasian Dosis (Glare / Dia equivalent), and Phlegma every 40 seconds. That's it. Toxikon II is so niche in its usage because it doesn't generate normally (limited to 3 at most if you never have to GCD shield) and doesn't have a purpose outside of double weaves/mobility. Double weaves are also because you want to use your healing skills -- which ultimately puts Toxikon II as a skill for your healing rotation -- which again brings up the previous issue. If you don't need healing, it doesn't actually have a place in your GCD rotation. Pneuma is also in its healing rotation and you don't use it under your GCD rotation every 120 seconds. Ultimately, 2 of a SGE's "GCD skills" are relegated to a binary conditional. In terms of diversity, SGE doesn't have it much better because Phlegma is 2 charges -- so there's a point where you just want to hold both charges when possible to dump it in raid buff windows now that all raid buffs are aligned to 120 seconds. The majority of the time is still spent on Dosis.
What puts SCH arguably above WHM is not due to its GCD rotation, but it's due to its engagement with Energy Drain optimization, as well as a more well-designed toolkit for encounters (mitigation checks only make SCH and SGE feel better). GCD Rotation only matters if there's enough to create a diverse and satisfying gameplay. SCH doesn't have a GCD rotation outside of Broil / Biolysis (Glare / Dia equivalents), but can be rated similarly to the both of the healers above exactly because WHM and SGE's "GCD rotations" doesn't feel as meaningful to its engagement. SCH instead excels at micromanaging excess healing resources they don't need through Energy Drain -- which provides a more meaningful engagement to its gameplay. It's still rated as a 5 out of 10 on average though, and that's no surprise because there's nothing else that's engaging outside of healing (which, again, is not always applicable).