There's no such thing as real magic, real witches, or real warlocks. All these words refer to imaginary concepts. So when English-speaking culture uses the words "witch" and "warlock" specifically for imaginary practitioners of malicious magic -- and, with all due respect to modern neopagans trying to redefine the words, this really is how they were and continue to be used -- that's the end of the story. That's what witches and warlocks are. There's no "truth" for the speakers to be misrepresenting or mistaken about. Saying you don't put much stock in that definition is like saying you don't put much stock in unicorns having one horn.
If that general fact about how words work isn't enough to convince you on its own, consider that warlock in the most literal sense means "oathbreaker". The war- is related to the ver- in verify and means something like "truth" or "trust"; the -lock means "liar" and (believe it or not) is from the same root. This is not a name one calls a person who is doing things of which one approves. It's as if there were a class called "Traitor" and you were telling me that characters of this class don't actually have to do any betraying.
That italicized quotation in my last post about corruption and destruction? I didn't just make that up. That was straight from the PHB. Again: the difference can be whatever you want it to be in your campaign, but that's not the difference that's in the core rules and you can't expect it to be the difference in future content based on the core rules.
A lot of the domain abilities actually do stuff like that. Cool things happen whenever a tempest cleric deals thunder or lightning damage, for instance.
By "Don't put much stock in that" I meant it the same way as "I don't put much stock in the Salem witch trials." The fact that magic doesn't exist is irrelevant to the point, which is that all sorts of people disapprove of all sort of things, and one person/group of peoples name for that thing need not be true. There are people
now who will call a doctor a warlock, the Devil, or all sorts of other mean things.
I found your quote, specifically only in the Fiend section. That doesn't at all apply to all the patrons, though I am happy to accept that my particular version of Fiends is far from the norm. The other two patrons are listed as inscrutable, or beyond understanding. At best, that hints towards their alignments being chaotic, but not by any means evil. GOO patrons might even be Unaligned, as far as their awareness stretches.
Aye, I read the cleric class, and would of course use it for inspiration on my rewrite. I wouldn't want to shut out those who are happy with the cleric as it is after all. I just want a deeper level of customization, and the Warlock scratches my itch for just the right amount.
I mentioned my specific reasons for wanting to change the cleric, but will mention them again here as they are relevant to the next point. I feel that, for a Champion of the Gods, few of their powers are actually different. Looking over the Cleric with a careful eye, I think my main issue is Channel divinity turn/destroy undead. That single ability shows up in 6 of their allotted slots for progression, and does nothing interesting. I would probably cut it out, or slide it down to one of the domains, to make room for more Domain specific Features.
I would also think about adding a third layer to customization, like the Warlock has with the Book/Chain/Blade. There have been multiple stories about clerics, or at least people who fight the infernal side of things, with different ways of showing those powers. Rituals, holy weapons or relics, or advanced knowledge all come to mind. I would need to think of good ways to represent these that do not mirror the Warlock features. The Weapons in particular would be an issue, as it basically comes to mind like a Divine version of the Blade pact.