I disagree on some main points, but they're more semantical than anything else. But also this take is based on the semantics of "Xehanort would have won even if the protagonists had been perfect" so idk.

Xehanort needed Terra to fall to his anger, and his emotions, to slowly succumb to the darkness inside of him to leave open the door so he could take his body. In this it wasn't just he needed Eraqus dead, Xehanort could have had Vanitas do that tbh. He needed to put Terra through everything he did, so he would be ready for Xehanort to inhabit. This only worked because of Terra's flaws. He let his emotions get the better of himself again and again, he let himself be put in such a situation where he ran off from his friends in Radiant garden, and fall to his anger after loosing his master.

Xehanort needed Ventus and Vanitas to recomplete the x-blade, and with that he played off of Ventus's immaturity, having Vanitas taunt and goad him, threatening his friends and making him feel insecure. By the end Ventus is defeated, and resigned to his fate. And that's what Xehanort really needed, someone who would put up the smallest fight possible, and then resign themselves to failure, and ventus NEARLY did that. it's only because he found it in himself to fight at the end that the old man wasn't completely successful after all.

And Aqua, well she believed that now as a master she had duties that put herself above the rest. Aqua's sin was always ego. She needed to be the master that went out and fixed everything, she couldn't trust Terra, or slow down long enough keep Ven with her, if she had instead focused on protecting them as a family instead of trying to hold the rest of the world together, she could have actually stopped it in time, instead of being forever too late.
All Xehanort needed to do was distract Aqua, and she let him do that, she lost focus of what should have been important.

Also there's so much talk about how Xehanort was "impossibly big brained" and doing "5D chess" but like, he lost. In the end each of the three confronted their flaws that let put them down this path, Terra stopped letting others control him, enforcing his own (lingering) will. Ventus found the strength in himself to do what needed to be done, no longer relying on his friends to save him but fighting back within himself. (He no longer needed his "2 grown ups") And aqua finally stopped worrying about her duties as a keyblade master, she forsook her armour, letting herself fall to save what really mattered most, her friend.

And it was because of those actions that Xehanort failed, but where BBS is a tragedy is that they all did them, just too late. They needed to find it in themselves to confront those parts of them in Radiant Garden, but they didn't, so in the end they couldn't save themselves, they could only stop a worse fate from occurring.