It's original within the bounds of a street fighter game. No other mainline street fighter game that isn't a crossover has had tag team.
New mechanics are important to keep the series fresh. And there is plenty of new ground to be explored, other series are able to introduce new mechanics every game so SF should be able to as well.
Most other fighting games are not focused on footsies the way SF is. So certain mechanics that negate focus on footsies, wouldn't work in a proper SF game.
I think you're also ignoring the subtleties in what mechanics SF6 is bringing back from prior games. Parries in SF6 function differently from 3rd Strike because in 6 they're complemented by the Drive system and you can't air parry in 6 like you can in 3rd Strike. Drive Rush off a parry or guard may seem similar to Guard Cancels from the Alpha games, but again they have different functionality in 6 because of the other systems they interact with, that weren't present in the Alpha series.
The speed definitely doesn't not look as fast as V. There is a metric shit ton more screenpause on things like drive impact and drive reversal, fall/knockback floatiness, etc.
On certain things like jab strings you can tell 6 is faster than 5 in part because there is no stupid three-frame buffer window in 6. Look at footage of Ryu players doing three crouching jabs into a Special vs., say, Urien doing 3 crouching jabs into EX Headbutt in SFV; the former is faster.
The speed's there in 6 where it still matters, with cancels & links for instance. Hit pause on EX attacks isn't noticeably more than in 5 from what I've seen, I think there are just some ways REEngine handles animations that UE4 did differently, and maybe you are conflating things on that front.
One of the few jarring things with 6 IMO are the grab tech animations; they look like they "snap" from the grab instance to suddenly the characters being far apart having tech'd. I think SFV's animation for that was better, same for 3rd Strike. I don't remember how SFIV's grab tech animation was like. I think for 6 they just need more transitional frames from the point of the tech being engaged to the characters actually pushing away.
Same SFV just feels so clunky compared to IV and surprisingly enough VI somehow manages to surpass V in that regard.
I kind of disagree with this. IV had a few big issues IMO. Visual confirmation for jabs and shorts was barely present; you (literally) got the bare minimum visual feedback to tell if your jabs or shorts were connecting. That combined with another big issue in IV: over-reliance on one-frame links. A lot of the best combos in the game, even a lot of intermediate ones, required you do one-frame links, and lots of them. It was just a huge barrier that IMO felt arbitrary.
3S is usually considered one of the, if not THE, most complex of mainline SF games, and it didn't actually rely a lot on one-frame links at all. Yes, some characters had big combos that required them, but they didn't dominate the meta in the high-level play the way they did in SFIV. 3S also didn't require reliance on techniques like plinking; that's another issue with IV and it pretty much made playing with anything but an arcade stick impossible at high-level play, especially if you were playing characters like Gen or C.Viper (who both relied a ton on one-frame links and tech like plinking, negative edge etc.).
I do like IV's speed though, and I do wish V and VI at least had speed options so you could adjust the speed to match it. But that would probably screw up the balance in frame data.