Atomic swaps have happened for quite a while actually. I am part of the testing team at Komodo Platform, where we have been actively swapping around 80 coins for a couple of months now. They have built a decentralized exchange, including decentralized orderbooks, decentralized order matching, etc. An easy to use GUI is in BETA now.
Some articles that probably explain it better than me:
https://blog.komodoplatform.com/everything-you-need-to-know-about-atomic-swaps-and-how-komodo-is-advancing-the-technology-cadaec50da7cThe topic where the current swap protocol is explained and 'invented':
https://bitcointalk.org/index.php?topic=1340621.0And the whitepaper covering all the details about the decentralized exchange BarterDEX:
https://github.com/SuperNETorg/komodo/wiki/barterDEX-Whitepaper-v2Thanks for pointing me to that software, will try to test it. A question arises: Would it be possible to use SPV wallets like Electrum for atomic swaps? If yes, then the storage problem would be solved ... Another idea for mass adoption could be if "hybrid semi-centralized" wallets like blockchain.info included an atomic swap feature, as they could manage "chain handling" for end users.
Merchants that want to accept multiple cryptocurrencies and convert them into Bitcoin for e.g. paying taxes in Bitcoin will probably have fairly significant disk storage requirements.
If SPV wallets are not possible, Bitcoin 0.11+ style pruning could be the solution for that - a merchant then could easily accept up to about 100 currencies with ~1GB storage use for every chain.
Swaps between two Electrum nodes have been successfully done for a couple of weeks now, too.