Sirlin pretty much covered exactly this case. The issue with DDD's infinites appears to be pretty much exactly the same as the issue with Old Man Sagat in his example - essentially, allowing him/them makes several characters pretty much unusable by anybody who wants to win. Or so we think, since we don't have 10 years of tournament experience with a several-month-old game.
It feels like the issue is more the unbalanced character matchup it creates rather than the infinite itself. The IC's have infinites on everybody, but a lot fewer people take issue with that, because the ICs overall end up fairly balanced, with the power of the infinites being balanced out by the fact that they're really hard to start up and avoidable, and by the fact that other than that, the ICs kinda suck. If DDD had an infinite on DDD, I don't think there would be as much of an issue with letting DDD dittos become all about that one grab - I, personally, would laugh at him getting a taste of his own medicine
So perhaps the better way of approaching the problem is to rethink how we let people pick characters instead of trying to go into the game mechanics and micromanage how you're allowed to play? If you rephrase the issue as 'if character X has ONE extremely bad matchup, it becomes impossible to safely use character X more than about one-third of the time without running the risk of ending up in an unwinnable match', it might be simpler to approach. This was less an issue in Melee, because there weren't as many characters that 'could be quite good except for that one absolutely horrible unwinnable matchup'.
The simplest way I can think of is just to allow either player to opt for a ditto (or maybe to opt to take Random?) instead of [whatever matchup] - allow the person to take the disadvantage of trying to beat the opponent at his own character instead of the greater disadvantage of being stuck in one of those matchups. Or give each player one chance to 'rechoose' characters, essentially giving the opponent the advantage of counterpicking characters to get out of the disadvantage of That One Bad Matchup.
Or something else, I'm sure there are other ways to set up the character select to give those Unlucky 7 a way to [usually] get out of the matchup, ones that don't end up changing too much else, the ones I listed are just ones I thought up off the top of my head right now, I'm sure there are better ways of setting it up, maybe even make it matchup-specific ('A DK vs a DDD is allowed to switch to ___' or soemthing).
Not like my opinion on this matters, I'm neither a smash expert or a tournament organizer, but just thowing some things out there that you more knowledgeable people may be interested in discussing