It looks like they've got that covered: "If we, what3words ltd, are ever unable to maintain the what3words technology or make arrangements for it to be maintained by a third-party (with that third-party being willing to make this same commitment), then we will release our source code into the public domain. We will do this in such a way and with suitable licences and documentation to ensure that any and all users of what3words, whether they are individuals, businesses, charitable organisations, aid agencies, governments or anyone else can continue to rely on the what3words system." -- https://what3words.com/pricing/
When someone told about what3words, they also told me that they made money by selling shorter or more pleasant word pairings to whoever was willing to pay, but the only reference to that I could find is [0]. Having that requirement would make it much more difficult to build such a system.
Apart from that, it's basically "just" geohashes with a different (population-density based) encoding.
