Chernobyl lacked many security features that are standard on all modern reactors. The biggest flaw was it had no containment structure. That's the steel & concrete dome which, if the worst happens, keeps the melted uranium contained within it.
That alone would have made the Chernobyl disaster just a bad local industrial accident, and that's the main thing I mean when I say it can't happen again. Though it's far from the only security improvement modern reactors have over Chernobyl.
> But that doesn't mean it's impossible
This is the "you never know!" argument. The good and bad part of it is the same: It is always true! Any thinkable disaster, can conceivably happen, and maybe you "should" be worried about it.
Of course, applied consistently, it would lead to total paralysis. This is no way to rationally approach risk.
Its still kind of a bad question,because we can't rule out dinosaurs attacking the power plant. Maybe the question should be,is there a less than 1 in a million chance of a level 7 event happening when using a modern nuclear plant design in the next 100 years?
If the original argument was "the exact same thing that happened at Chernobyl will happen again", then that's meaningless and irrelevant, because... who cares? What people actually care about is whether a nuclear disaster could happen again, one that kills a bunch of people and makes a significant area of land uninhabitable for some long period of time.
If you think that's "moving the goalposts", then I don't think you're here to have a good-faith discussion about why people are worried about nuclear energy.
Having said that, I do believe that a significant accidental nuclear disaster is much much much less likely now than in Chernobyl's time. But that doesn't mean it's impossible, or that we shouldn't think about or be worried about it. And also consider that's "accidental": we also need to consider the possibility of terrorist- or state-level attacks, which may be harder to protect against.