Yes, that's true. I just think they wouldn't be such pushovers as long as all you have to do is know about Type advantages. Even when all they're trying to do is test you, you'd think the test would be a little more difficult for most of these guys.
It would be more believable if each Leader also had a battle tactic they were trying to make you aware of, teach you how to use, and teach you how to counter it as well.
That's right, but the difficulty issue mostly comes from babysitting gameplay design.
If I had to imagine gym battles in a "realistic" fashion, I'd asume they would be actually fair battles, with Set rules, no potions/revives, and both challenger and leader using the same number of Pokémon.
Leaders could be made a lot harder by forcing competitive rules onto the player, giving leaders held items and better teams and strategies. But developers don't care, games are designed in a way to ensure that even players who know nothing about Pokémon can beat them. They also want to make sure that everyone can win using their favorites, even if said favorites suck as a team. Games even let you bypass a difficult battle by just overlevelling, and leaders won't scale accordingly.
I agree that the concept of gym leaders feels a bit outdated. I think it would make more sense if leaders were based on a theme, a certain fighting style, or particular strategies, rather than just "I use Rock Pokémon because I love Rock Pokémon so I refuse to use anything else"