It is partially open-source, as I understand it, the core OS is, but some UI components on top etc. are still closed source.

https://docs.sailfishos.org/Services/Development/Sailfish_OS_Source/

Jolla and SailfishOS inbound

Also unsure about that, but it is true that Google help fund its development (unless they stopped?), and I would also believe it its just to help themselves.

What would actually make it succeed though? I have been using it for a long time, not like there are any issues.

Just seems more of a culture for people just use Chrome. Is marketing the solution or what?