I personally think no, though you're right in the regard that it's going to offer a nice alternative.
Even if you are able to swap between digital assets, there will still be significant demand coming from fiat <-> BTC trades which simply can't be handled on a decentralised platform, or at least, not yet.
You could argue that stablecoins can be used, but not everyone is willing to deal with the risks that are associated with using a token that is potentially based on fractional reserve, so completely replacing traditional exchanges will probably be out of the equation.
We're still early for this, since the LN hasn't even been used at a large scale within the mainstream world yet. Not to mention, it's not production-ready (rather unstable). So, it may take a long time before we experience atomic swaps in mass. In the meantime, we have decentralized exchanges which suit their purpose well from exchanging one crypto to another.
The biggest downside of decentralized exchanges though, is their lack of liquidity and popularity within the mainstream world. Also, it's not widely possible to trade from Fiat to crypto without submitting ID documents (like in the case of OpenLedger). But, in this regard, centralized exchanges are better by a long shot.
Personally, I prefer to use both centralized and decentralized exchanges for my own specific needs. Whenever I want to preserve my privacy, I would use decentralized exchanges or atomic swaps. But if I want greater liquidity, and convenience/ease of use, then I would make use of centralized exchanges.
Nonetheless, it's becoming certain that centralized & decentralized exchanges, along with P2P marketplaces and atomic swaps will co-exist within the future. This brings many alternatives or options for people to choose from depending on their daily needs. Just my opinion
