I agree. It is also funny that Rowling built her school story in such a way that it's completely believable that everyone could be ended by a madman walking in with a gun. It's like she lives in the States.
House elves liking being slaves isn't a plot hole it's just an odd way to portray slavery. As for how the book got popular, providing basic wish fulfilment and escapism to a generation of readers who had nothing better to do than to fixate on that world.
Ah yes the old trick of house elves naturally being able to perform advanced wandless magic that wizards can;t even perform and still ending up as slaves.
Technically even this could not be a plot hole with some extra explaining. Maybe house elves are only good at utility spells and aren't good at offense or defense. Maybe there's a cultural reason why they allowed themselves to be enslaved in spite of being stronger. I doubt Rowling bothered explaining the houseelves' odd talent for magic in context to their circumstances, though.