Contrary to what people seem to assume, you actually can change the schema of a database and migrate the existing data to the new schema. There's a learning curve, but it's doable.

If you go schema-less, you run into another problem: not knowing the past shape of your data. When you try to load old records (from previous years), you may find that they don't look like the ones you wrote recently. And, if your code was changed, it may fail to handle them.

This makes it hard to safely change code that handles stored data. You can avoid changing that code, you can accept breakage, or you can do a deep-dive research project before making a change.

If you have a schema, you have a contract about what the data looks like, and you can have guarantees that it follows that contract.