However I'd like to point out that that is exactly the reason why DuckDB relies so heavily on its extension mechanism, even for features that some may consider to be "essential" for an analytical system. Take for example the parquet, json, and httpfs extensions. Also features like the UI you mention are isolated from core DuckDB by living in an extension.
I'd argue that core DuckDB is still very much the same lightweight, portable, no-dependency system that it started out as (and which was very much inspired by how effective SQLite is by being so).
Maybe some interesting behind-the-scenes: to further solidify core DuckDB and guard it from the complexity of its ever growing extension ecosystem, one of the big items currently on our roadmap (see https://duckdb.org/roadmap) is to make significant improvements to DuckDB's stable C extension API.
disclaimer: I work at DuckDB Labs ;)