I think the general feeling in the particle physics community, at least, is that the next ten years or so in particular are critical. By now most of the theories in vogue now will have filtered through to the general public. In no particular order, beyond what comes to me first, they include Supersymmetry, (Walking) Technicolour, Extra-dimensional theories, various others such as "little Higgs" models, a few others that have been bouncing around for a while. The overriding theme of pretty much all of these theories is that they are expected to kick in somewhere within the scale that is about to be probed at the LHC. Thus, if they do not turn up -- which is a real possibility -- then we are at a bit of a quandary, and new ideas really will be needed. One or more of them could still be seen, of course.
In fact, that is the justification right there. The entire future of the field that has spent much of the last century or so advancing humanity's understanding of how our world works at the quantum level is either about to hit a brick wall for the first time in quite a while, or (to use another cliched phrase) the floodgates may well be about to open onto discovery after discovery. Both of these possibilities are exciting, although in different ways.
But then the point of diminishing returns is that they are far from diminished yet. Probably not even close -- or at least, hopefully not. There is still a huge amount to explore, and rather more than just 0.01% of physics. Whether we can or not with a feasible amount of money is another matter. Again, whether or not it is worth bothering probably depends on what is turned up at the LHC. If we find something "new", then it's probably justification to go to higher energy scales to, to really understand what is going on. If not, then I think there's a strong case for not just building an even bigger one (able to go to 100TeV, say, or about seven times more powerful than the current one), and instead it might be time to pause and explore a new direction.