i dont think it kills any later on floaties. they can still get the same ideal DI for survival even with the angle reduction. before, with a 50 degree angle, they wouldnt have really wanted to DI the full amount anyways (meaning, DI to achieve a maximum angle change of 18 degrees, which would send them at a 68 degree angle in this case). if they DIed to that angle before, they would be DIing well past the corner, and would would be sent past the blast zone earlier. . Now, even with the angle reduction, they can still achieve the same, ideal amount of DI (to get them aimed at the corner) that they had before.
I can see this change being a really good buff against fast fallers who ideally would DI to achieve the maximum angle from the horizontal. now the limiting angle for them has been decreased for them, making their survivability lower.
You're right - with proper DI, this change makes no difference for floaties, because you can get to the corner either way. But given that most people probably aren't DIing this properly, in a lot of circumstances it'll likely kill
slightly later on floaties if/when they DI up. In the end, it's not a substantial "nerf" to off-the-top kill power, and even then, it's more than compensated for by it being easier to hit now.
But yes, the main buff is against fast fallers, since it's tougher by a little bit to DI, and punishes worse DI by a good bit more.
After playing with it a little bit (most of my time has been spent with Bowser, to be honest) it does seem to be a much more flexible move now. It's much closer to Marth bair in general utility, as Azure mentioned, since it's fast and now large enough to actually cover a decent area behind him. When you're in an awkward position with someone behind you, bair is now actually useable as a quick getoffme or a light tap to get that little extra percent or positioning, where previously you could only do a behind DED, which I think is a lot slower, and doesn't get them off you nearly as well/can't get you as much position.