While healing can be hard to balance it is far from toxic and is not meant to fit the same role of a shield. Shields are strong instantaneous protections that soak damage so that you don't take damage, they are strong during a fast trade but are overall weaker since they do not give you sustain and usually don't last too much. On the other hand healing abilities are not meant to block damage for you, they are meant to give you the sustain you need to get back on your feets after a trade or to help you sustain through a fight, they are usually weak during fast trades but are stronger for poke and sustained fights.
You are comparing 2 mechanics that don't fit the same role, they might both reach the same result (saving your life) but in different scenarios (one during fast skirmishes the other during drawn out fights and/or after the fight is done). Again heals can be quite hard to balance but they are not a toxic mechanic, the champion who are built around them usually are since they tend to be spam bots with little to no interaction with the enemy. Take Alistar as an example: his heal is weak but spammable and he has to stay close and personal to the enemey else he's useless. Is this toxic? Nope, he has a lot of interaction with the enemy and his heal is a marginal part of his kit and also costs quite a lot compared to how weak it is, then take Soraka: she's the definition of a 0 interaction spam bot. Why do bots usually play with champs like soraka/sona? They're the less interactive champs of the game, it's easier to make a bot for them and to get away with a passive behaviour. Is this toxic? Yes there is just no counterplay to her since she just doesn't interact with you the whole game, her only interaction has the range of the whole lane and you can't do anything about her other than dodge her Q spam, also her heal has a low cd and is strong especially compared to the low cost (same with Sona and Nami). On another note Janna is also quite toxic since she's like Soraka: her only interaction has huge range and she can just stay back and shield the whole laning phase. Is she centered around her ult's heal? Nope, she's centered around shielding and disengaging, does that mean every shield and disengage is toxic? No, there are a lot of shield and disengage that aren't.
So tl;dr: while I do agree that heals are hard to balance and making them scale off of ap is just fine (champions with heals are quite strong early but can fall off easy mid game if they take no advantage, only to rise up again in team fights thanks to theyr strong utility), they are far from toxic.