I've been using Redex for many years - before premium fuels became available - and I can assure you it's not that simple.
It's fine if you always run the fuel tank completely dry before the next filling-up...
I have often found myself at the pumps at a motorway services, trying to guess how many litres of fuel I have left in the tank, calculating the amount of additive needed based on the recommended ratio, and then trying to work-out how many milliliter is each notch on the bottle... without my reading glasses and often not with the best lighting conditions.
Not to mention fumbling with the bottle and funnel and then having to put the funnel away in a plastic bag to avoid the strong naphtha smell spreading in the car.
Not an insurmountable task, granted, and one which I have done for over 20 years, but it was quite a relief when premium fuels became available I didn't need to do that any more.