Traditional Highland dress is certainly not a costume, nor is it categorically unappealing to the younger demographic; I'm a young man and I like the way it looks.

What I think the style of dress in my OP provides is a contemporary alternative to the traditional mode, not a replacement. Most of the dressy occasions I have worn the kilt to (weddings, graduations, etc), I've been the only one or a very small minority. A more muted colour palette combined with colourful tartan seems to fit well and be welcome. I'm inclined to reserve more traditional outfits for Highland/Scottish/Celtic gatherings.