Please scroll up and look at my previous comment about “adversarial interoperability”. You’ve actually given a great example of why W3W is a bad idea for most use cases: Excel. In the post I linked, MS Office is given as a prime example of a company with a stranglehold on a market, and the damage that did. Apple reverse engineered the Excel format (and Word, and Powerpoint) and wrote their own software that could open the same files, and I don’t think anybody would seriously argue that we’d be better off today if Microsoft had held on to their desktop monopoly.
The difference with W3W is that you can’t just reverse engineer the product and make a compatible competitor. The database content is the product, so to make a compatible service you’d need to reverse engineer (i.e. steal) it. That would be a copyright violation, and thus by definition they cannot have compatible competition. As you point out, with any other system, customers would have the option of bidding out to a competitor, because the data — the addresses they’ve encoded for their postal system or whatever — is in an open standard. Only W3W creates vendor lock-in, so customers have to either keep using W3W, even if costs increase enormously or performance is terrible or the whole board turns out to be a bunch of serial killers, or completely start over with a different addressing system altogether.