Permanent walkoffs are still a problem even with almost all forms of infinite and chaingrabs removed, in my opinion. This is for the following reasons:
1) They promote degenerate, campy gameplay. Irrespective of the actual risk of blastzone camping, it can actually be a good strategy if you are playing to win because it makes approaching for the opponent an insane risk as well. This is less of, though still an issue to an extent, on temporary walkoffs like Castle Siege especially as the second transformation lasts long enough for this type of gameplay to become optimal and thus occur, which has been seen multiple times in tournament settings. We don't have footage of permanent walkoffs because, well, they're banned, but think about what happens on Castle Siege's second transformation and translate that to extend throughout an entire match. It's not pretty.
2) They completely remove ledge play. It doesn't really matter how the game was designed or how the developers view the game or whatever. The bottom line is that competitive Smash as the competitive community plays it has had ledge gameplay as a major part for a long time, and it's a valuable part of the game that makes it competitive and fun to watch. This is something that other fighting games don't boast. Edgeguarding is an incredibly deep and exciting part of the game that has become even more complex arguably with the addition of ledge trumps in this game. Walkoffs remove this intricate level of depth.
3) The ability to knock characters off stage and edgeguard them is a significant part of competitive balance. Let's take Little Mac, for instance. He has an amazing ground game, but he's done when he gets off stage. Captain Falcon has speed, is a heavyweight, is great racking up damage, and has insanely powerful kill moves...but he has a linear, exploitable recovery. Even Diddy Kong for being such a great character pre-patch and still solid even still would sometimes struggle due to his relatively subpar off stage game and recovery that really is just good enough to get the job done. Characters with bad, decent, and good recoveries generally have those for a reason. Walkoffs completely ignore this aspect of character balancing. I'm not saying Little Mac would become top tier due to walkoffs being legal or something; that's not my point. It's more that these stages completely ignore any detriment in recovery because you never have to recover and completely change certain matchups, like for instance most vs Falcon or Little Mac. On a normal stage with ledges, I can kinda get pushed around a bit by these characters, but edgeguarding makes dealing with these characters much less of a burden.
4) Most walkoffs have one of two issues: either they're way too big for horizontal KOs, or horizontal KOs are way too easy due to the smallish size of the stage. Both of these are rather intuitively undesirable.
5) Despite the fact that we don't really have infinites that go across the stage anymore, we still have strings that can carry players across the stage into death on walkoffs. One example is say a Sheik low percent fair string carrying someone off stage. Normally, this is fine because the opponent is at a low enough percent to wear Sheik's moves may not necessarily have enough knockback to pull off a successful edgeguard just yet or the blastzone is far enough from the ledge that actually killing off these strings is impractical. Ledges also limit these strings much more because the Sheik player can't get the tight timing to continue chaining fairs with no stage beneath her; it simply doesn't work that way. On walkoffs, though, and especially vs heavies, Sheik can sometimes just fair chain the character into the blastzone, especially if they're anywhere near the edges.
These issues make the legality of walkoffs an impracticality to me.