I don't interpret Zoro's promise to be entirely literal. I've always seen it as his own personal affirmation to never feel like he's lost. By that I mean, he'd never let his spirit be broken after defeat, to never feel like his efforts were in vain just because of a set back. Of course, he goes into every battle not expecting to lose, but if he does, he'll just work 10x harder than he did before. And he does so not just for himself, but Luffy as well. He is metaphorically Luffy's sword. And as his sword, he should be ready to strike down anyone who gets in his captain's way.
What I have contention with is when someone suggest losing would be good for his character. What they are bringing forth is just a retread of the Mihawk fight. All losing to someone now would do is confirm who is better than Zoro, nothing more.