If those scaling ratios are too low to be of use, then maybe they're due for a buff. Things that have no purpose should not be in the game. If the AP scaling on a champion is there as nothing more than a cosmetic, then maybe it shouldn't be there at all.
Healing and Shielding power IS awesome. You know what else gives extra Healing and Shielding power? More AP. I'm fine with those passives staying there as a means to compensate for somewhat lower AP on a support item, or to enhance Warden's capacity to do these things. Believe me, when i'm throwing around 50% MS buffs and 300+ heals as Sona, the team notices. Support kits were being unnoticed because they didn't have the income to make their kits BE noticeable, so Riot turned to actives instead.
Let's look at Nami for example with 600 AP; She has a 20% AP scaling on passive MS buff, so an 180 MS burst everytime she does anything to an ally, a 30% healing scale on W that also gains 7.5% increased efficiency for every target jump per 100 AP (including damage) meaning she'd heal/damage for 365/530 scaling upwards 30% of those values every jump, her E has a 20% damage scaling and a 5% slow scaling, so 3 AAs worth 205 damage that slow for 65%.
Actually looking at these numbers for once, i'm pretty impressed at what she can do if she scales. Normally i only look at Sona. You can't tell me these aren't great supporting values though. I know you arguably have Mikael's most of the game at full efficiency while the skills scale to this point, but i'd say it's offset by the skills having a cooldown lower than 10 seconds.
Also, CDR and Mana Regen would still very much be on the support items, since what i'm calling for here is the augmentation of power and cost of support items, and removal/nerf of several item actives on the roster, not transition from support itemization to mage itemization.
As far as i'm concerned, double utility from items is a non-issue. Midlane mages would still sacrifice the burst of Ludens and the safety of Rylais and Zhonyas to gain access to passives/actives like Censer and Mikaels that they potentially can't even use properly, even if they were strong enough to warrant a buy. Another very simple way to prevent stacking is to make these passives unique and unstackable/detrimental to use more than once, like the Locket active currently is.