Earlier quoted context omitted.
The balance is probably around 90% the car and 10% the driver. In the years of RB dominance, a subpar driver like Webber almost got the championship. OTOH Alonso lost a decade in Ferrari and McLaren while being, tied with Hamilton, the best driver in the grid. The 10% of what the driver adds to the mix is what makes Hamilton champion instead of Bottas.
The ratio varies - I would say it depends on the car, teammates and team. But ultimately driver skill is what determines who gets to drive for Mercedes, Ferrari etc. So once a pairing has been made, either the drivers are closely matched, or one or both will be changing teams or leaving F1 all together.
That's not always the case. Eddie Irvine spent 4 years as n.2 to Schumacher pretty happily, only changing teams because Jordan offered more money. He was pretty spectacular on good days but nowhere as good as The Michael.