There was an interesting analysis done that compared the final seasons of shows with the rest of their seasons.
Only one TV season *ever* that has lasted 3+ seasons has had a larger discrepancy between its final season and the average of the rest than Game of Thrones- House of Cards, which has a darn good reason for its last season being dogshit since they had to fire the main actor due to sexual assault allegations.
Game of Thrones was a great show, I don't like this excuse that it failed because its expectations were so high. I don't think that's it at all. That would explain a slight drop, nothing like this freefall.