You can't make someone see something they refuse to see, or believe in something they simply don't care about in the first place.
You can try to explain to me how a small party will inevitably not be able to comfortably handle all the skill checks that appear everywhere in the game, which makes the game less enjoyable, all day, it won't matter if I simply don't care about making skill checks, and won't find the game any less enjoyable if I fail skill checks everywhere, as long as I can still progress the main story.
You can try to explain how a small party greatly limits party composition choices all day, it won't matter if I never care about party composition or role-playing with a group of characters. I will be perfectly content if I can blast through the game with one single character. Experimenting with different ways of tackling encounters using different party compositions? Pfft.
You can try to explain how playing with more characters is more fun and interesting, it won't matter if I find it more enjoyable and easy to just have one single character to manage.
We can drop all the arguments as for why we believe one thing is better than the other. At the core it's just "because we *prefer* it that way". It's just personal preferences, no matter how absurd or stupid you may think someone's preferences are. If I enjoy playing with one character because I'm too lazy to care about the nuances of playing with 6 characters, none of your explanation will matter to me. I have the right to not be one of those sophisticated players.
"We make our choices and take what comes and the rest is void."