In previous editions, paladins could fall and lose their class abilities. We know this, because the rules said so. Rules As Written.
But in 5e, such rules are conspicuous by their absence. The lack of such rules =/= these rules exist RAW, because Rules As NOT Written are not Rules AS Written.
They're only conspicuous by absence if you put them in the wrong context. If you look at the AD&D rules and compare them to the 5E rules, then you might draw the conclusion that paladins could fall in AD&D and can't fall in 5E.
But AD&D rules aren't 5E rules, and it would be wrong to take them into consideration here. Just by what it says in the PHB, about how the game works, the DM is already fully expected to role-play the gods to the best of their ability. We might reasonably disagree about whether any given action is something that a particular deity could or should do, but both perspectives are equally RAW, as long as both of our opinions are based on what it actually says in the book.
My own best guess is that they took out the specific rules for falling, because they didn't want to seem like they were encouraging that as the only option. I've heard stories of bad DMs who would contrive moral dilemmas that were nearly guaranteed to make a paladin fall, because they read those rules in the book and assumed that's the sort of thing they were supposed to do. By not explicitly calling out the possibility, it leaves the DM free to address egregious cases, but it doesn't encourage them to pull the rug out under a well-intentioned player.
And here is an assumption: the player is deliberately playing wrong! But in my experience what happens is that the player and DM disagree about the best way to role-play their devotion. In real life religious people, even of the same religion/denomination/church disagree about religious matters, and each still goes on happily being a member of that religion. But the DMs I'm talking about say it's their way or the highway, taking away the player's agency.
There are players who intentionally make a mockery of the setting, by playing to the letter of the rule rather than its underlying intent, and banking on the expectation that the DM won't call them out for it. That sort of thing happens all the time.
In my experience, jerk players are significantly more common than jerk DMs. If I have one message for anyone reading this, it's that the DM needs to stand up for themselves and their game. Don't let a player use the rules to bully you, because nobody at the table will have any fun if the DM isn't having fun.
Yet, on this forum and in real life, the knee-jerk reaction of some DMs is, "Paladin/Warlock? The player MUST be making a mockery of the story and the PC cannot possibly make sense!" Ban, ban, ban!
The DM is the only one who could possibly know with certainty what makes sense in their own world. If their immediate reaction is to assume the player is up to something with such a suggestion, then that makes sense, because it is likely to be the case most of the time. If the DM has a reason to believe that the players is well-intentioned, then they may choose to look further into the matter, and then make a decision.
It is unreasonable of a player to bring such a character to the DM with the expectation that it will be allowed. That's just pure player entitlement.
