Without an MEL everything on the aircraft must be serviceable before strictly legitimate flight.
That's your opinion, Mr Hamster.
It's not the legal position for private flight in a certified aircraft, whose required equipment etc is defined in the POH.
It may be that if everything installed is only what is in the POH for "day VFR" then all of that must work for any flight. But most planes have a lot more stuff in them than is required for Day VFR, either in the POH or by the applicable airspace regulations. The "extras" don't need to work.
The Cabair example is a redherring because they are an FTO and are bound up in all sorts of ways, even if not doing charters which need an AOC anyway.