A stock exchange which clears trades trustlessly without frontrunning and hfts is a compelling case for a dex usecase. The assets that people care about have on/off ramps which instantly create value in those assets proportionate to the level of service those ramps can provide.
Better clearing, yes I agree with that. It is a niche application that won't ever matter to people outside the industry, but nevertheless it is a significant opportunity. As for decentralized exchanges, I'm not so sure they don't increase opportunities for front running by being slow. When you talk about HFTs those are situations where speed of light delays matter. People have tried to build non-real-time liquidity pools before and it hasn't really caught on. So we'll see, but I'm not convinced it is compelling.
Even so that's largely inside baseball for traders (whether professional or otherwise) and wouldn't be of much interest to the Average Joe or Jane by any means. What ArticMine is talking about seems closer, potentially.