I really like the idea, it clicked in my mind when first hearing about the space platform, that you know item in space wouldn't stick to the belt, so it would need to be enclose, but that risk being just pipe. I like your drawing, it is not just pipe, although there existed pneumatic tube system which were really just pipes for items, i think you managed to still make it look like clearly an enclosed belt.
Although now seeing your drawing, i realize the main drawback (to me) would be that it doesn't convey the idea for inserter picking things from and putting things down on them. Hiding items while being a problem if there is mixed item, is already there when you use lots of underground belts, you can't see where the contamination is spreading/has spread, so i'm not considering it too much of a problem. But such (nice) drawing, seem like an "upgraded" version of the belt, like their space counter-part, which would require some special insertion and extraction point, instead of being wide open for pick up all along.
That feels like taking a string, and pulling on it and you never know when it's going to end, because to me that's a whole new system which i think would be fun to play with but is no longer the same as a belt, it is certainly inspiring !