Something I haven't seen mentioned yet (if it has and I missed it, I apologize)...

The "clan tartans", as I understand it, weren't really clan tartans until after the clan system was pretty much destroyed. Before that, people wore whatever tartan they happened to like.

Why would the future be any different? Why should it?

The entire discussion has revolved around the concept that clan tartans are (or should be) kept for use by Scots who are affiliated with a particular clan. But it wasn't that way to begin with, and at this point I really don't see it ever going back. That particular genie is long since out of the bottle. Assigning significance to a tartan is a relatively new (post-clan-heyday) concept.

If we're reaching into the future, though, to project what we'd like to see, I would very much like to see the kilt become more commonplace in society with the new tartans gaining widespread recognition for their significance. In 30 years, let's say, I'd like to be visiting another US State wearing a Texas tartan kilt and have people know that I'm from Texas. Will that happen? Probably not. But one can dream.

Another thing Jock Scot mentioned is the question of why anyone would have more than one tartan. If we are to go back, again, to history where all tartans were fashion tartans (i.e. people wore what they liked), the same question could be asked of anything you wear. Why have more than one color shirt? Why have more than one color pair of trousers? Why have more than one color pair of shoes? The answer should be obvious. People who aren't wearing tartan as part of a uniform will want diversity in their wardrobe.