Tekken has always used the same ranking system with small changes. Similarly, I believe Soul Calibur has as well. I never really played SC4 online, but I think SC5's ranking system was an iteration on that. That said, I think it unlikely that SC6 will change its ranking system to resemble Tekken's. I wouldn't want them to anyways; Tekken's ranking system is kinda volatile and frustrating in that regard.
But here's where the ranking system lineage gets interesting: Pokken's ranking system is an iteration on SC5's ranking system with key changes to make it less exploitable. That is to say, you can't just grind it to hit the top. I'm an okay player, but I really shouldn't have been able to hit A1 on XBL with my fundamental flaws: this was a failure in SC5's ranking system at multiple points.
Anyways, how Pokken improves it. For one, advancing point-wise toward promotion thresholds is much more difficult. Wins are rewarded less, losses are punished more. Especially when you lose to a rank lower than you, you lose like three times as many points as you would have gained if you had won, and twice as many points as you gain from beating someone the same rank as you. Pokken is more frustrating than SC5 in this regard, but at the same time, it definitely does the job whereas SC5 kinda didn't. It was too lenient and thus exploitable. Especially since you could see the rank of someone matched up against you and turn them down!
Next, promotions and demotions are gated by tests. In SC5, once you hit a promotion point, all you had to do was win one game against anyone C5 or higher. In Pokken, you have to win a certain number of games in a set in order to promote, and the tests get more difficult as you get higher up in terms of having to win a higher percentage of games. The things that most help are that it is typically more stringent with matchmaking, so you're going to get people who are actually a match for you; and you can't turn down ranked matches in Pokken, so you have to play them. Promotions are actually earned. Demotions are gated too; you don't just bump down if you get a bad streak and hit the threshold; you get a certain number of matches and typically just have to win one of them.
Lastly, and this makes much less difference than point distribution and promotion tests, there is an extra S tier after A. The main thing is just that this is extra room to grow and differentiate upper echelons of play. All of this together means that the S ranks are actually comprised of an elite few. Pretty much all of the high-level Pokken players I follow are in A tier, with a few in B. I only know of one North American S-rank tournament player.
Anyways, I would really love to see Soul Calibur 6 iterate further on Pokken's excellent ranking system.