Your C permit is not valid until 2028, only the document/physical medium is.
A C permit is permanent and does not need to be renewed (just need to redo the document, like an ID card/passport).
Unless you leave the country of course; for more than 6 months in the C permit case; then in this case you can ask to put the permit on hold for up to 4 years.
If no citizenship is granted, once RAV ends and if I don’t find a job, can they force me to sell my flat and leave Switzerland?
No, you are a permanent resident now. Your permit does not depend on your job anymore. There are provisions for revoking a C permit, but as far as I know they only apply in case of serious crimes.
And even if you were to leave the country, you are not forced to sell. Edit: it appears to be more nuanced, after reading the thread below.
Also see this thread: