What is the difference between a paladin and our cleric of Moradin(or any other martial focussed cleric)? That in the end the cleric has wasted all of his shots at customization just to look like the paladin while the paladin is still free to pick any theme and background he fancies. (And not many people has really though of the inverse, why have clerics if a Paladin with the right background and theme can look a lot like one?)
Also remember THEMES AND BACKGROUNDS ARE CLEARLY OPTIONAL and very likely to be in the hands of the DM who may simply turn them off to get an old school feel, should that happen the cleric that claims being a paladin by virtue of background/Theme stops looking like one, while the Paladin witht the paladin class is still a Paladin.
If the only way to have a paladin/ranger/druid/bard/barbarian is by picking a specific combination of theme/background/class they are going to have lots of unhappy players that will just vote with their feet and go back to 3.5, 4e, 2e or PF where those are classes. If there are is no paladin class then we have no paladins, period. No matter how many can be argued how backgrounds and themes replace them, we must be able to play a paladin without backgrounds and Themes, they are a pluss than can (and will) be turned off, not something vital like the classes.