Re: Oh no!
"To all available evidence, we have got all the particles down pat, there is bugger all up to the Planck scale"
That seems to me to be the real problem here. For every previous request for a new accelerator there has been a fairly plausible case that it will see new physics. By fairly plausible, I mean there is confirmed, existing, old physics that demands the existence of some new physics in the target energy range for it all to make sense. As far as I know, that's not the case this time.
"Stringers can go shove it."
Or, paraphrasing, these theories *may* be correct, but there are no existing observations that require the effects of supersymmetry or string theory to start being visible at energies of "just a little bit more than last time". Give that these new effects have roughly 20 orders or magnitude to hide in between LHC and the Planck scale, and even with unlimited cash we (*) couldn't hope to probe more than half a dozen of those, perhaps it is time to stop the accelerator game and find more subtle experiments.
(* We can't, but of course Mother Nature has access to considerably more violent machinery than we do, so if you have several billion to spend then the cosmologists are more likely to make the observations you are looking for.)