> you cannot expect to make things better than they are at the input of your pipe segment

Not in the short-run. But the reason working on the pipeline is valuable is that discrimination includes negative feedback loops. When people who would be entering the pipeline look for role models and success stories and don't find any, they're discouraged from trying. So while there are of course diminishing returns in investing in a dry pipe, if one is interested in changing the status quo, it's valuable to keep the investment a bit above the break-even point to encourage flow upstream (flow that, one must remember, will take half a decade or so to show up as a pipeline increase, depending on how long you measure the pipe to be).