You clearly know a lot of what you are talking about, so do you think Marth will still be top 20 or no, and do you think he will be overall worse than in SSB4
Marth was one of the few characters that were nerfed. Sakurai's team clearly treated him like a top tier on their initial balancing campaign. There's been a significantly nerfed Jab, DB, DS, kill power, and damage.
The other characters did not generally see a reduction in damage, which meant the 1 v. 1 damage bonus made them look like powerhouses. Marth however, does comparatively less damage to his major damage-racking tools. This would be his aerials, DB, and the lack of Jab combos. Marth also lost: throw combos and aerial strings – two other major damage-racking tools – but other charactets saw those too.
Knockback reduction is seen in Fair (and possibly/probably more aerials), Usmash, and DB so far.
To summarize, nerfs were made to his main approach options, damage-racking tools, and kill options.
HOWEVER, the game changes favor him more than other characters. In the last three games, Marth had a garabge initial dash length/dash to shield, dash turnaround, and dash stopping. The first one made him historically weak to projectiles. Because of these, we had to do things like resort to walking, which was good but had slow acceleration – so no boost mobility there. Assuming everyone's initial dash now is uniform like rumored, these dash changes benefit Marth far more.
Then there's the landing lag. Marth's aerials have always been fast, disjointed, and long-range. The nearly halved landing lag from most characters, including Marth, highly benefit Marth – giving him free reign in the air (like Melee/Brawl) and powerful approach and pressure tools. This is primarily why all the top players that have tried the game said that they like him.
The most important thing to mention is that Marth is a simple character that is almost unchanged from Smash 4 when you look into the grand scheme of things. For those that used Marth, all that skill will transfer. For those with good spacing and funamentals, it's easy to pick him up. For those last two points, the fact that Marth's design always fit competive Smash, and the dash changes...
I think he's not as good as Smash 4 Marth
But I think top 20 is just barely possible.
I'm guessing this means that if they DI correctly Marth can be punished for using DS?
At low percent, if they DI down and tech roll inwards, I think he could be punished.