What's 'modern paladin?' If the distinction between paladin and cleric is that the paladin is much more fighty than the cleric who is more casty, I think paladin could easily work as a fighter subclass. Basically an eldritch knight except with cleric spells. And sure, that would make paladin a bit less magical, but to me that is a feature, not a bug.
I think the problem is you are not looking at all the paladin class features that aren't spellcasting and weapon/armor proficiency: Divine Sense, Lay on Hands, Divine Smite, Divine Health, Aura of Protection, etc
Mechanics and Flavor, the modern paladin is neither a fighter nor a cleric. The paladin doesn't defeat challenges by casting spells nor using martial combat techniques. The paladin uses divine boosts to overwhelm hostile opponents and their charisma to sway potential allies.
The paladin is overtly magical. The paladin however doesn't solve its problems with spells. The paladin itself is the magical being. In layman's terms, the modern paladin is literally playing a monster. You're not a warrior or a white mage. You're a lesser angel/devil.
You can shove the 2e paladin back into the cleric or the fighter. Not the 5e one. That's the real reason why a rollback won't work. You'd have to strip the fighter or cleric bare to load the 5e paladin back in OR create an overpowered reward subclass like the old paladin.
I mean, the 5e paladin is one of the strongest classes in 5e. Do you really want to give it full fighter or cleric features on top of that?