That's.... not what the poster was saying. Prestige TV extends much farther than the mafia genre, sopranos was just the turning point when higher budget dramatic things were more commonly greenlit.
The Wire, Breaking Bad, Watchmen, The Expanse Game of Thrones, The Walking Dead, Mad Men etc all absolutely blow X files out of the water. And while I do love X files, the monster of the week format is not even in the same ballpark as those long form character dramas.
I think you might be letting nostalgia blind you to how great television has become.