I guess this debate is more about personal tastes and opinions how games should look like (based on the games we've played) and not whether Kon's mockup is actually playable or not.
It's an uneducated assumption to presume that people are just trying to conform Alex' mockup to preconcieved notions about what games look like. Personal opinion is a factor, but things like positive/negative space, layout, and presentation of information are staples of graphic design. The debate is about whether or not they are relevant. I would argue that in video game art they are not only relevant but the priority.
can you spot any tiles which a person can mis-interpret and furthur more die from in my mockup? i can't. the grass tiles are pretty clear to me that i can stand on them, and the dark grey tiles are pretty clear to me that i can walk in front of them. you dont need neon yellow caution tape on everything.
The grey tiles and the grass appear to be on the same plane to me, and they both seem solid since they're similar levels of contrast to the other positive spaces, and very much separate from the sky which is the only obvious negative space. This is easily fixed by just separating the dark gray from the light gray a little more. As Rosse's edit points out, the layout is complicated to begin with, so some visual assistance is necessary. You can call it "caution tape" if you want but if someone on my team were to tell me "it's the viewer's responsibility to discern what's solid and what's not", I would call that lazy design.
Anyway it's your art and no one is paying you, so have it your way. But your work will suffer from an attitude like that.