Schemaless design is a tradeoff like many things. There are systems where being able allow the client to write new data structures can be important. (one example I've seen was a system that tracked arbitrary metric messages from devices that could add fields based on their device type. The system needed to be forward-compatible without speaking to the parties implementing those devices).

Also, some of those systems have a _dynamic_ schema, e.g. Elasticsearch, where they learn the data type on the fly, but disallow incompatible changes on fields already known, which is a bit of an inbetween.