For portability and durability of your data I find that "File over app"[1] is more important than open source. These are all separate vectors:
- VC vs user-supported
- Files vs databases
- Open formats vs proprietary formats
- Open source vs closed source
- Extensible vs non-extensible
- Private vs privacy-invasive
An open source app can still be VC-backed, store its data in a proprietary format, have terrible APIs, and include telemetry.