Wasn't the nightmare scenario that the LHC would create a black hole that would swallow the entire solar system?
The case of no new physics "just" mean that they'll have to start looking else where. Their job just got a little harder, and while I can appreciate how annoying that can be, it's hardly a nightmare.
We know that the current theories aren't correct anyway. Or more accurately, they are correct only in low-energy conditions, and can't say anything about high-energy conditions.
This is "nightmare scenario" because we knew that the theories aren't correct, but we hoped for some results that would guide and inspire new theories. Either way we know that we aren't done yet, but the current state is like being in a featureless void, unable to know where to direct one's efforts.
...so we should expect a period of wild speculation in physics with some fairly interesting experimentation to prove (or falsify) those theories.
(I'm not a physicist), but is it possible to almost automate some of the discovery process to attempt to turn up new leads? (or is the search-space too large for this to be reasonable?).
The true nightmare scenario will become true when advancing from plateau of scientific advancements will become too hard for human minds. Maybe it's impossible for people to comprehend or navigate in ideas located above some level of complexity, the same way most animals can't notice themselves in a mirror. So far our progress has been quite impressive, but what if there's some sort of human "Moore's Law" for science, where further advancements without AI will be close to impossible?
Over the years I was slowly beginning to become convinced that the abstractions of relativistic model are just not compatible with all other models we have. Not negating the correctness of it's results, but I kinda think that it should be redefined so it conforms better with the rest of the models. Alas, it has become de facto a state religion and any attempts at discussion about it end up with being laughed at