Now that the Large Hadron Collider (LHC), a bigger particle accelerator in Europe, has come online, the justification for existence of Fermilab's Tevatron, which has been operating since 1983, is gone. This is a very long run for a scientific experiment in quantum physics, whose Standard Model was just starting to reach its current form at the time.
Tevatron has been instrumental in pinning down empirically the properties of the heavier particles in the Standard Model, in validating the Standard Model's accuracy, in ruling out a huge swath of beyond the Standard Model physics, and establishing the unexpected new physics of charge-parity violation in quantum physics. It has failed, however, for want of sufficient power, to locate or rule out with great scientific confidence, the existence of a Higgs boson (although it has provided suggestive hints), to locate or rule out the existence of new particles beyond the Standard Model, and has not established definitively (perhaps because there is nothing to find) any non-CP violation beyond the Standard Model physics or evidence establishing or ruling out Supersymmetry which is the core prediction of String Theory. The creativity of theoretical physicists has, alas, considerably outrun the capacity of experimental physicists to constrain their speculations.
The scientists at Tevatron have been pushing the limits of what can be discovered with an experiment that sized (cleverly and valiantly, it should be noted) for years. LHC should be able to easily examine high energy physics conditions that Tevatron has been using long data runs and brilliant experimental designs to probe for a decade or more, with sheer brute force, in a year or two.
Experimental high energy physics is mostly about probing very low frequency random events whose frequency is a function of the total amount of energy devoted to the experiment, and the more power you have, the easier it is to get data samples large enough to reach statistically significant conclusions about these very low frequency random events and formulate them as physical laws and scientific constants. There are some minor feature of Tevatron that make it more suited than LHC to explore certain kinds of phenomena, but we've already tried out all of Tevatron's best tricks, so the universe of science that Tevatron can do that LHC can't do passably well that hasn't been done already is pretty trivial. It is the moral equivalent of a strong telescope that has scoured the skies for everything there is to be seen when a much bigger one comes along.
The knives are out for federal budget cuts, and Tevatron isn't cheap, even to run once you've built the thing. The September shutdown date coincides with the federal government fiscal year. This means fewer jobs for a lot of exquisitely but narrowly qualified U.S. scientists and technicians, but since LHC publishes its results for free or something close to free, the American public will still get the benefit of its discoveries in almost real time, and of course, many American scientists do work at LHC.
Tevatron may be the penultimate high energy physics experiment. If LHC fails to discover any new beyond the Standard Model physics, or if its results lead to the formulation of a "Grand Unified Theory" that seems to provide a complete explanation of everything but gravity (something the LHC is ill suited to provide data upon) or at least provides strong empirical support for a theory that implies that there is no good reason to expect no discoveries at higher energies, then there is a very good chance that nobody will have the will to spend enough money to make a bigger successor to the LHC. Unless LHC has left us with some real tantilizing reason to think that there are more discoveries to be made with just a little more power by 2030 or 2040, we may lose the will to continue this brute force approach to experimental physics and the era of high energy physics will end.
This isn't just going to happen decades in the future either. Before either of my kids takes their first physics course in high school or college, the LHC will have pretty definitively determined that there is a Higgs boson where the Standard Model predicts it should be, or there isn't. If it finds one, and then finds no new physics after that, the inclination to call the Standard Model a done deal that can't be fit into some more grand plan as theoretical physics have dreamed of is going to be quite intense.
If the LHC doesn't find a Higgs boson in the next few years, the theoretical physicists across the world are going to have scratch their heads, throw out 90% of the theoretical papers that have been written in the last thirty years, and figure out what they've been doing wrong all of this time. This may call for new experiments, but may lead to the conclusion that we've been doing the wrong kind of experiments to find what we are really looking for with them, making LHC style big science less attractive.
Of course, if the LHC finds a Higgs boson, and then a dark matter candidate particle, and then indications of large numbers of new supersymmetric particles, or finds that a growing body of evidence shows that one of the Standard Model's lesser known component laws is incorrect (all the best known well as well established in a very wide range of circumstances), then the LHC may be swiftly followed by something bigger and better. We will learn that it is not the end of history, at least as far as fundamental physics goes, scientists everywhere will rejoice, and science fiction writers will imagine discoveries of currently impossible things without the least bit of guilt.
But, while we don't know which of these scenarios will come to pass, the laws of nature are already in place, so this outcome is largely a matter of destiny and fate at this point.