The fly fishing line market is very large, and the fly fishermen all have different needs. Some prefer the most durable and longest lasting line, while others want to get the most out of the line. My fly line needs to be the most durable, but at the same time have the most flexibility. It has to have a minimum of 10oz (300ml) of line, and it has to be flexible enough to allow for all types of conditions.
My line needs to be the most durable and flexible, but at the same time it needs to be the least expensive. I’m happy to pay more (like $20 for a 5lb line, $10 for a 20lb line, $75 for a 20lb line, etc.) for the best line, but I’m not going to go out and spend $400 a day on fly lines as long as it lasts.
For fly fishing, you need a good line that is very flexible. It needs to be very flexible to allow for a wide range of conditions, and that means that line needs to be very durable. There is an argument to be made that the two are not mutually exclusive. A good line is flexible, but it is also durable. A line is flexible, but it is also durable. A line is not flexible, but it is also not durable.