That is the pith of it all and a more simpler way to digest it. They are friends and because Usopp saw himself in Merry (lacking in a sense for the journey), he felt he was being slighted as well. His decision was to take it to an extreme and put that friendship in jeopardy.
If your friend took it that far, then decided to come back to you, would you be as open and accepting as Luffy was prepared to be? That was Zoro's point. Putting the friendship on the line shouldn't be taken lightly if he's going to return. Usopp should be considerate and reflect on what he did. When he returns, he should be able to easily show that he's sorry if he really did reflect on what he did. What Luffy was willing to accept was worthy of solicitude in this instance.
All the things he did to help the crew and Robin do not negate that he was willing to end the friendship like that. Luffy didn't create that wager. It was Usopp.
A relationship is 50/50. If you're letting someone that doesn't care for you as much as you care for them, walk all over you, then it's not a friendship. Giving your energy to someone that doesn't return the same is unhealthy. Can't afford to do that on such a turbulent sea.
Yeah, as much as I would have personally let Luffy go to Usopp with the understanding that their conversation might have played out in their own way, without Luffy letting Usopp turn into some heartless monster who’d threaten the crew’s safety and lead to it falling apart, I’m still glad Usopp ended up not letting his insecurities conflict himself anymore, especially in light of how he previously missed the chance to open up his heart after Nami opened up hers just before getting to Alabasta.