But, you've ignored the fact the Kaiilurker has limited us to core. He rejected expansions in 4e for solving the problem, so, why should expansions in 3e or 3.5 count?
One of the greatest problems with any edition break is that you go from a fully-realized system with all of the expansions, down to just two or three books with none of the expansions. There was no dedicated healing class out, upon launch of 4E, where the Healer class
was available immediately upon the 3.5 launch due to cross-edition compatibility.
And even if you're counting the fully-realized system in either case, there was never even a dedicated 4E healer
build, let alone an entire
class. Because of the way the AEDU structure was designed, you couldn't start a fight by buffing your allies, and then transition into healing when things got rough. There was no at-will healing/buff spell, and you didn't get enough Daily abilities in a day to fulfill that role. The expectation was that you would attack the enemies in order to buff your allies. Even the pacifists were expected to attack (with the only caveat being that they couldn't attack someone who was already bloodied).
Additionally, you're talking about a pretty extreme corner case. How often is a party getting hit by two fireballs in a row? Certainly not something that's going to happen every combat or even every session. And meager damage was a 3e thing, not an AD&D thing where the cleric was second to the fighter types in terms of damage output.
I'll grant that two Fireballs in a row was more of a 3.x thing than an AD&D thing, because 3E had the sorcerer, which had more spells per day and less spells known. If you're going against a sorcerer with a good Dex, who wins surprise, then you could easily take two Fireballs before getting a chance to react. Taking two Fireballs with only
one round to recover would be a much more common occurrence, and would show up just about any time you faced such a spellcaster.
Fireballs have always been a big deal, though, regardless of edition. Fireball meant that the priest was, at best, a third-rate damage dealer (a second-rate fighter, where fighters had great at-will and lousy burst damage). When the going gets tough, and it's time to break out the big guns, it's not entirely unreasonable to expect the priest to do what she can do best, rather than relying on the old standby.
Your experience with AD&D is going to vary a lot though, much as in 5E, depending on which rules you want to use. When I played, fighters did a
lot more damage than priests, on account of Weapon Specialization. That single class feature would triple the minimum damage for most weapons. I seem to recall that there were other supplements which allowed other classes to specialize, so that could have made a difference. If you tended toward higher stats, or more magic items, then that also would have brought them closer together.