Very interesting question! Off the top of my head, my view on this is that substance is always more important than style for film. That's why when we see a film that is all style and no substance, it is very noticeable and easy to criticise. Whereas films with no style and a lot of substance can still be a great watch. Also a lack of style often can aid the substance. Not all films need style or as you say "to look cool", but pretty much all need substance to be deemed good cinema.

It's like you order a gourmet burger in a lavish restaurant. It arrives and there's no burger, but everything else is there; the bun, cheese, dressing etc. You eat it anyway and it's a great bun, cheese, dressing etc, but you still can't help but feel it's disappointed as you got no damn burger. Now imagine you get none of the extras and just the plain burger meat. It's cooked to perfection and even though you didn't get all the other stuff the meat alone was enough to satisfy you. In fact you realise that having the extras with it wouldn't have even suited it as the natural juices were so good. What I'm trying to get across in this terrible analogy is that the burger meat (substance) is more integral to the meal (film) than the extras (bun etc) and sometimes aren't even required.

What must be considered is that simply a lot of style is neither necessary nor suitable for a lot of films. The first director who came to mind when I saw this post and the idea of "substance over style" is Michael Haneke. If you think of a film like Cache it's all substance. There's a lot of long static shots and the film is just played out in front of you. I guess that could be deemed "the style" of choice, but it certainly doesn't look cool like I think how you're envisioning style OP. This "lack of style" though if you can rather unfairly call it that, perfect suits the film's themes of class divide and family. It's also quite a slow moving film so the "lack of style" aids the storytelling in that way too.

I think it's unfair to call this a "lack of style". It's a style all the same and just because it doesn't have all the bells and whistles of a Guy Ritchie or Tarantino doesn't mean it can't be a just as, or a more effective way of telling a story. I hope I made some sense there OP. Basically substance is a cornerstone of good cinema whereas style or what I think you're seeing as a cool look is not always necessary but sometimes is. I just don't think there is such a thing as substance over style as substance should always be over style.