Defence, defence... DEEEEEEEEEEEEEEFENCE!
If a fighting game has a weak defensive system, it will always boil down to which characters have the best, most reliable or hardest to escape 50-50's, and how easily the now genre infamous "death corners" can be abused (i.e., in 2-D fighters). SFV suffers from this problem, and it renders the game a yawn fest (...compounded by the many 'crutch' mechanics and overall casual nature of the game, suffice to say).
Soul Calibur VI, although a 3-D fighter, is a good case in point for what NOT to do, with respect to said aspect of fighting games: A series that was lauded for and, indeed, built upon its robust defensive ("guard impact") mechanics, took to watering it down in what's generally considered the worst SC game, 'V', and then proceeded to not fix it in 'VI' (only dumb it down, perhaps making it even worse), leaving the game all but dead a month after release and little more than a casuals' spam and mash fiesta for those who persist with it.
Anyone who has ever learnt martial arts in real life, knows that defence is the first form of offence, and it is this aspect of combat that is first taught to would-be practitioners. After all, if one cannot avoid an attack landing on oneself, all the fancy kicks and flips in the world mean nada... when you're already in a coma!
Also, for those who blow the predictable "it's a video game, not real life" rusty trombone -- fighting games are fantastical representations of real world marital arts and, as such, must adhere to certain principles of said real world combat. Up still has to be up, and down must be down--if a game is to avoid descend into the realm of the absurd and losing all connection to its source material.
Without a robust defensive system, a fighting game's longevity lies -- as the OP put it -- right up until the "meta" is revealed; then... Fin.
