Seems like the argument is in the "Rune" part even more down below. I think it boils down to having specialized editors which you embed, and to a common interface that you may define over that. The point on ambiguity localization is kind of curious too.

I don't think there's an argument to be had for all structures, just that you can do it for each custom structure, and that's the point.

Maybe the editors of the old tried to bite off too much when they attempted embedded structures, and they didn't have the right abstractions in place. If you look at https://tylr.fun it shows that things that weren't being done back then, there are interesting approaches now.