Real food is not really a vague concept - it's meat and vegetables (plus eggs and dairy) - everything that can be eaten and is bought raw.
It doesn't mean that one has to eat only that -just that the more the better.
As for the coke - if somebody NEEDS his daily coke AND enjoys the taste of diet coke - I think it's a step in the right direction (though a bit risky as it might raise insulin levels).
But if he hates the taste - I'd say staying with normal coke but decreasing the amount would be more beneficial, as he won't feel like he's giving something up, which would cause additional stress - and we want to minimize that when changing eating habbits.
I believe that what you're trying to say is that for every person with given restrictions (life habbits, budget, goals) we can propose a different diet and relatively this diet will be better - and I agree with that.
What I disagree is that you're going a bit too far with that - because in absolute terms this diet will still be worse than the "best" diet following the guidelines I'm pointing out.
The thing is - it's fine. Your diet doesn't have to be perfect. But to make conscious food choices we have to know on which part of the "healthy" spectrum we are. And that's just it - awareness is key, and by saying "every diet is good if it works" you are one step too close to denial/ignorance.