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.
Maintaining backwards compatibility for reading old records in code is not hard. You can always rewrite all rows to the newer format if you want to remove the conpat code, or if the structure changes in an incompatible way. It's pretty comparable to what you have to do to evolve the code/schema safely together.
Having schema is much better for ad-hoc queries though, doubly so if your schemaless types aren't JSON (e.g. protobufs).
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.