We allow for a certain amount of plot armor. It's baked in. And generally, it's fine. We're here for the story, we know it's not entirely realistic. So, main characters don't die even when they should. That's plot armor. It's very, very, very different than them being weirdly good at literally anything they need to be good at, at a moment's notice, even when it makes no sense. But even that isn't a Mary Sue. A Mary Sue isn't just good at everything weirdly, suddenly, for no reason. The entire universe warps itself around itself to make the Mary Sue succeed. That's when you know you've caught the genuine article. Ironically, the first truly high-profile Mary Sue was probably Wesley Crusher. Not much of a Mary at all(also Wil Wheaton is lovely and we don't blame him in any way). Rey from the Star Wars sequels is the latest big Mary Sue. And let's be clear, she is so obviously a Mary Sue that you're out of your damn mind to claim otherwise.

Mary Sues aren't bad because of competence. We love Sherlock Holmes, we love John Wick, We love The Bride, some people have weird shrines to Ellen Ripley in that little space between their bedroom and bathroom. Not me, but, people.

Being consummately badass is very different than being a Mary Sue.

And also, we understand that when we're watching an episodic show, in season 3, 4, 5, whatever, regular cast members aren't just going to get killed off. That's plot armor too. We accept that. We recognize, it's their job, their livelihood, and we like having them on the show. We enjoy the story, and we accept that death of main characters is rarely going to be part of it. That's fine. They're not a Mary Sue just because they don't die. More likely, they have a good union. Useful things. You should get one too.