What's the threat from Snowflake and Fivetran to dbt Labs?

Nothing prevents warehouses or Fivetran from offering dbt or other transformation technologies. They already do so. dbt is open source, so they can offer simple dbt runners.

For our users, many don’t want the business logic of transformations and metrics to be as tightly coupled to the warehouse or BI layer. Many customers have multiple warehouse vendors or multiple BI tools, so you don’t want to reproduce the logic in different storage and consumption layers. In a lot of ways, our abstraction and vendor neutrality makes dbt so powerful and useful to consumers.

The other reason why people buy dbt Cloud instead of using a dbt runner in a Fivetran or another EL product is because we offer a far richer overall product experience. The people who do transformation work are often different from those who do set up the extract load flow. So, it's a separation of tools that match a separation of responsibilities. The transformation layer needs to be closer to the business needs versus the pipes of data movement, which is what Fivetran or the EL provider owns.