Sed does not have a debugger that is 50 years old. The author was not saying that sed is a bad tool; he's saying it doesn't have a debugger. If there is a desire to have a debugger for the tool, that has existed for 50 years without having one, perhaps the tool is not being used in the way it has been used for the last 50 years. This would seem to make the argument that the tool has been tried and tested less relevant.