If Zoro hadn’t stepped up, what would Kuma have actually done to Luffy? Based on what we know about Kuma now, it’s hard to believe he would’ve taken Luffy’s life. I think, instead, he might have separated Luffy from his crew if he felt they weren’t strong enough.
In my opinion, it wasn’t just Zoro and Sanji stepping up that saved Luffy. I believe Kuma already had no intention of harming him. Their bravery may have simply convinced Kuma to let the crew stay together. Even if they hadn’t stepped in, I think Kuma would have sent Luffy somewhere like the Revolutionary Army’s headquarters or another island where he could grow stronger.
After all, I don’t believe Kuma would ever kill Dragon’s son, no matter what orders he received. In the end, Zoro’s actions are what kept the Straw Hats united at that moment, but I don’t think Kuma would have broken them up for good. That’s just my take on it, of course.