"I was able to spam endlessly without ever running out of mana at that cost."

That was the original intent of Cata healing.
The point was to have all your spells matter in some manner.

Your basic heal could not run you OOM. But it was only suitable for moderate damage with plenty of time available to cast.

Your quick heal was there to get someone out of the red zone fast. It would become necessary if all you used was the basic heal.

Your slow, heavy hitter was a mana hog, but if someone was not in danger of dropping quickly, AND they need a lot of healing, it is the best heal to get them topped off.

AE heals are, well, AE.
HoTs are involved in here as they can slowly heal people so they fall in between the basic and big heal.

Well, now that is gone and instead of having skill matter for knowing when to switch to one of the other heals, you are in a race against the clock. Who needs a boss enrage timer when you can expect every heal to suck down that blue bar more than anything they have to bring it up? "Do I heal that DPS, or do I save that mana for the tank since I am low? Guess the DPS is on his own."

Is a DPS that sits there with no resources fun? Is it okay to have one that can't do a simple rotation without running out?
Is it okay to have a tank sit there unable to do anything because they ran out of resources while tanking?
If the answer is no to those 2, why are healers special?

I loathe healing. Because every time I hear about how they want to make encounters more exciting/fun/dynamic it tends to translate to "hose the healers somehow" and no one minds. So now they make healing more fun by doing what? Oh, making healers the only role in the game that will have limited resources, will be unable to effectively regain resources, and will have to make decisions based on minutes later.

I forsee any DPS with self-healing to be at an advantage in this setting, because then your healers can sit around on their hands for a little while and get some mana back, or maybe cast a few dozen lightning bolts...

(If this double posts, sorry, I have had a few posts swallowed up recently and I am not sure why.)