Latitude/longitude and OS Grid references are free and, because their construction is public domain, will remain so for ever. If one supplier tries to charge me to get it, I can switch to another.
W3W is not public domain. It is operated by a commercial concern whose interest is in making money not in providing a public service. The assiduous way they protect their product from becoming public domain confirms that they intend to operate commercially.
The serious concern is that if they can succeed in becoming the de facto geolocation app, if they can embed themselves to the point where we've all become dependent on them, they can then start charging us all - and we will have acquiesced in privatising something (geolocation) that ought to be a communal resource.