It is the false claim for a non-existent history that has earned them their infamy, not the fact that they designed new tartans.
Exactly.

But one wonders what would have happened if they hadn't foisted their fraud upon Scotland. Would the tartan craze ever have happened? And even if it did, would the people have gone back to wearing random tartans instead of trying to identify themselves with 'clan tartans'?

It's easy to criticize the Sobieski-Stuart brothers, knowing what we know today. But despite their being fraudsters, at least they did provide a commonly-accepted definition for clan tartans. It gave everyone a starting point, so to speak. Without them as a unifying force, there's no telling what would have happened (or not happened).