As some of us have heard, Pat Symonds was famously quoted that the 2014-20 power units have such a wide power band that five forward gears would be enough. So why haven't the rules been changed in the three years to open up the number of gears for choice, instead mandating eight forward gears?
If it's to push efficiency, the three extra gears (which must be rather hefty even though it's F1; they are transmitting torque in the four-figure numbers) are just added weight, (thus burning more fuel) and the teams having to compromise (because the ratios are fixed) results in a ratio set that is always suboptimal for the circuit they're currently on.
Furthermore, less gears could likely make for a faster car, as those 75 ms gear changes add up. The advantage is probably miniscule, as we don't see teams gearing a five-speed car and carrying the remaining three gear pairs around without using them. It could also improve reliability, as the gear changes are when gearboxes snap.
Finally, a reduction in number of gears could open up a possible return to true manual gearboxes, improving the show, and putting more emphasis on the driver, all without reducing safety.