I'm not actually sure Thanos CAN kill Steven. At least not without like... trying to wear him out over the course of hours. The level of abuse he's just kind of effortlessly parried or withstood is pretty up there, probably not incomparable to the Hulk space whale punch.
Movie Spoilers. And while I'm not sure Steven can hurt Thanos, Obsidian probably can. (Assuming Thanos doesn't have any gems yet, that is.) Edit: Wait, just reread the OP, he's got most of them already. Never mind.
So Steven will certainly get a chance to talk. And... I dunno if it will accomplish anything? Like, Steven can infuse lifeless earth with plants, and can thus probably provide a lot of food to worlds whose fields are going barren. But as Totoro hints at, Thanos's plan never actually made sense and so having an alternative presented to him seems unlikely to change his mind.