I kinda feel like everyone talking about how bad Link is in previous smash games isn't envisioning how development goes and how dynamic balance can be in that environment. Link being bad in the previous three games (though he's really not THAT bad in Melee) is just irrelevant to the question of whether he could be overpowered in smash 4; it's easy in development to make any character brokenly good or uselessly bad, and final balance is just your best attempt to make it all work together to avoid both those extremes. Among other things, we don't know how else he's been tweaked, and we don't really know just how powerful this change is.
By my intuition, it doesn't seem very strong. Link using dair off-stage is a desperation move at best (landing it and coming out ahead would require the hardest of hard reads), and by the description, it only works as a meteor right at the tip right at the instant it hits which makes it quite tricky to use as a juggle escape too. I can see some possible strong utility in a more offensive situation like while doing some grounded pressure; meteors against grounded opponents have always been super advantageous openers, and Link probably has the tools to cover it if he played it smart though I'm not sure what kind of follow ups he might have especially with how committed it tends to be (just hitting twice with the dair would be fairly underwhelming I feel though certainly better than nothing). It's really hard to say just how viable that whole idea is or whether there's some other way it could be good that I'm not thinking of, and of course, you have to think across match-ups (like it's probably not a big deal off-stage in most match-ups, but maybe it helps Link a ton against a few characters with easy to intercept recoveries). Sakurai saying it is subject to playtesting is good since it's entirely possible it is too good for reasons that haven't been seen internally yet but may be later, and it would be a shame for him to commit to it being in the game now only for it to prove to be a bad idea. Of course, him showing it is a sign that, while he's not 100% sure on it being a good idea, he's reasonably confident since it's nothing but bad for him to get people excited for something they'll never get to use.
This was a great picture either way; honestly seeing this kind of clever idea for balance changes to old characters excites me more than any other kind of picture. There's a lot to think about here, and I'll probably be thinking for days of random situations in Brawl in which this sort of thing might have changed how things played out. I could go until release happily without any more new character or stage reveals if every picture were like this; here's hoping that the last two this week continue this strong trend.