I'm probably being a bit slow, and i admit i don't know paladin healing that well, but apart from FoL, HL, HS and dispell when needed, what the hell do you want him to cast? I'm supposing he's using Divine Favor and Lay on hands when and if needed.
Also, in a 5 man dungeons, in Legion, 12 years later, 6 expansions later what extra spells do paladins use? I imagine there should be plenty! Or maybe there's only a couple, maybe there's only 2 or 3 12 years later, and 1/2 of them are things that are pretty much cast and forget like beacon.
So, my point? Obviously you had less abilitys, obviously you have more now, maybe you should have even more now considering we're 12 years and 6 expansions apart, don't try to oversimply stuff to make a point you're trying to make believable.
If i wanted to go even further, and despite as i said not having played a paladin myself, i'd wager that a decent paladin would have 2 or 3 versions of FoL and HoL ready to use to save mana when needed, and if we take those into account we'd probably conclude that they would press more buttonz than now, but as i said previously, and many others also, the difficulty doesn't come from here, it comes from the tunning that was im some, many cases more demanding than the average contemporary content.
Obviously some classes/specs, specially dps ones, were more on button ponys than the others, even more taking spell resistance and immunity from mobs/bosses taken into account, but this for me just makes it a better game tbh
. Why would a mage be able to zerg a fire elemental with fireball? Why would a hunter or rogue be able to poison a mechanical unit? It just makes sense to me, probably doesnt to you, but it makes to me