You still haven't listed a single option that wasn't already present in Brawl. Saying there are no polarizing matchups this early on means nothing. Though you should know infinites for Smash 4 on some characters have already been discovered so there goes that theory.
Who is saying there is a new mechanic in Smash 4? I can't make an argument about a BRAND NEW FEATURE because we both know there isn't one. Smash 4 improved on Brawl's mechanics and balance and fixed many things, and that is what makes footsies so different this time around. The changes have changed so much about ALL of Smash 4 - not just footsies, but aerial combat, off-stage play, and so forth.
THIS is the argument.
If you are not accepting this as the argument, then that's not my problem. If you will ONLY accept a "new mechanic" as a counter-argument, then why should I continue?
I'll continue anyway, because I'm a masochist.
Infinite jabs in brawl (useless) now have finishers. You cannot DI out of the majority of multi-hit moves anymore (making many moves actually useful now). Stage spikes require tech-input or else you bounce. Ledges do not infinitely provide invulnerability. Tether-grabs have less miss-delay. Knockback is lower at low %s generally across the board. Flying isn't a thing anymore. Dash Dancing was not only risky in brawl, but slow and telegraphed. Acting out of pivots/dashes was very different. Shield Breakers make shields less safe in general, and many characters possess the ability to pressure shields and threaten it to break, or just break it outright with a special or custom. Brawl has chaingrabs, Smash does not.
How are any of those things not good answers to your insisting premise?
Heck, the change to chaingrabs is a big deal on its own. Grabbing is not as powerful as a result, and we end up having a footsies game that focuses more on tilts and smashes, and less on grabs. You can't just end a footsies battle by initiating an infinite grab-based combo. Characters who could chaingrab became much better at footsies in Brawl, for example.
In Smash 4 the stage is more even as 'being able to chaingrab' or 'being able to force trips' or 'being able to stun' or 'being able to attack with infinite priority' is no longer a requirement to compete in footsties battles.
You can actually be a generic fighter in Smash 4 and do well due to the way your attacks work and flow together, or how strong your smashes are, or how you can punish enemy whiffs.
In Brawl, the metagame developed with top-tier characters being ones with gimmicks. Falco had his CG-spike, Dedede had his, ICs had their chaingrabs, MK had his general OP priority attacks, Snake had his dominant DACUS, Diddy had exclusive multiple forced ranged trips, ZSS had very powerful stuns, etcetera etcetera.
All of these things have been changed or removed. In Smash 4, there is a LOT more going on when it comes to the footsies metagame, and it won't boil down to gimmicks as well because the developers actually tried to avoid those things this time around.