Thanks to everyone for your input!
Streak, what motivates this question is when I try to swim a fixed interval at the outer range of my speed capability. Since that is my goal, it is almost a given that I will encounter fatigue at some point in the interval no matter how long that interval is. The question then becomes how to deal with it.
grahamsellers, I agree with you entirely. This process can be viewed as a kick in the pants. If you are up against it and have to deal with it, it forces you to become creative, figure out the causes and then try to find workarounds. This is the motivation for the questions I am asking.
I know that TI preaches letting speed come to you, instead of the other way around, and I have followed this strategy for some time. Eventually I got the sense that I had plateaued doing this. When I started pushing myself, I started to learn new things and this got me interested in the process. Perhaps this is because, when you push yourself, the sensory input becomes more intense and easier to understand, perhaps it is just (see above) that I needed a kick in the pants. My sense is that all of these strategies have something to offer, but if one uses them exclusively then eventually the well runs dry and it is time to try something else. Switching strategies also keeps things interesting.
Today is Thanksgiving in the USA, an American holiday. Yes, I am very thankful, especially when I think about the turkey dinner I will have later today, but I am also saddened by the fact that all of the pools near me are closed until next week. I wish Happy Thanksgiving to everyone!