I think what queen told sanji is important
If you can generate enough force by using a certain amount mass
You can't protect yourself
Haki enhances the ability of the user but it doesn't negate all things that aren't haki
Like a crazy example would be
Let's say that fuji can bring down a moon lol
Onto big mom
It wouldn't need any haki to kill her
The sheer amount of force would crush and turn her into mush
Even the logias can be harmed by their natural enemy
Sand / water
I really do think that it's an aesthetic thing that bothers people when it comes to kidds fighting style
It's all about crushing and suffocating which is pretty violent and brutal
In a comic like one piece it's harder to really emphasize what he's doing without getting kind of gruesome
Oda focused on haki for so long that I think the community perception became
You're either a haki master or you're useless
Which I don't think is true
But to each there own I guess
And I think the only way to keep big mom away from the roof is to defeat her
She can ride on promethious or hera if need be
And they're no longer over a body of water