Using a player from the top echelon of the Japanese competitive scene to illustrate character balance is disingenuous and, quite frankly, dumb.
Wait, there's another super cool game that limits your choices (but that's totally fine since you can always beat the paid characters with the characters that you do have with just a teeny bit of effort) - it's called League of Legends!
Lurker here. It's funny that you bring it up, because I ended up getting Kinda Decent at League of Legends at one point in my life and there was a moment around Season 2 / 3 where Xin Zhao, an Oriental-themed brawler character was an absolutely horribad champion. He had lackluster base damage, weak crowd control, unimpressive ultimate and was overall a shadow of his former, pubstomping self, completely outshined by the powercreep that was already settling in in that game and crippled by nerfs. Characters like Nocturne, Riven or Renekton offered pretty much the same thing Xin did (at the time Xin had no niche at all, and brawler design was quite a bit homogenous - basically, slap a cleave attack, a dash, some sort of stun and some temporary-massive-stat-boost ultimate, and call it a day), but better.
And then Riot Games decided to nerf him further specifically because he was dominating Bronze and Silver ranks.
We're not talking about a character like Shaco, who was a very high-risk assassin whose entire power was frontloaded to the first 8 minutes of the game and relied entirely on getting your lanes to snowball so that achieving victory was beyond doubt. He was quite unique in that regard; most other characters in his role brought in a ton of utility so that if their ganks and control failed or didn't secure a decisive advantage, they'd still be a fantastic enforcer, buffer or protector.
Yet Shaco had plenty of dedicated mains that got him to Diamond because of strong carry potential in solo queue and a pretty decent skill floor/ceiling even though Shaco, for the most part, was a pretty weak character as well; a D-tier character whom, if somebody picks him in solo queue, you pray to whatever higher power exists that they are actually skilled enough. Xin Zhao was a tier
below that - a character so utterly gimped that he was unusable if your opposition was capable of
breathing. To fall victim to a Xin attack you pretty much had to be blind to his approach and then just allow yourself to get whacked at, as many characters were able to simply outduel him.
Oh, and said Shaco was also nerfed along the same lines, though there was enough to his gimmicky design for people to still play him even if you had to put in a TON of effort to play him, compared to your run-of-the-mill ganks-at-level-2 jungler. Just because he was "unfun to play against" when you're not skilled or coordinated, but otherwise a very, very unremarkable, "cheesy" character.
Needless to say, experiences such as these don't make me believe that you're right. I'd understand if the example was FAB pubstomping some casual netplay lobbies with Potemkin, purportedly a weakling in the roster - but this was an example of a top player standing his ground against other top players while widely considered to have a handicap of sorts.
Point is: if you don't leave balance to the top players, you end up with balance dictated upon a whim of the Flavor of the Month. Also, for all the faults of the same game that you're dissing for having the power creep tied to unlockable characters, it also generally allowed you to gain access to solid, in-meta characters on the cheap all the time, and you didn't have to spend a dime on any characters. If Gouki was released in the LoL model, you'd have to probably win like 100 games with Ryu or Chun-Li to get him, and then he'd get nerfed to Dan levels after the massive outrage on the forums.
I agree with Nathir that the DLC is more likely to be there for introducing fan favourites and, as regrettable as the practice is, it probably won't affect competitive scene.