The fundamental challenge of open-source ETL is that high-quality connectors require understanding and working around all kinds of corner cases in the API of each data source. It’s very hard to get open source contributors to do this kind of work; it’s a real slog. Hence at Fivetran we’ve always stuck with the commercial route.
A customer perspective from a mid-stage CFO: I like saving money, but prefer to pay for software solutions like this directly. I pay you, and you make sure this set of connectors {in and out} continues to durably work. Meanwhile, our engineers can focus on building our product.
Open-source doesn't mean you can't have both. You can check how Databricks or Confluent are doing.