We do not know that this is forever. Natural Selection pressures which lead to the development of larger forewings my over the next few decades lead the butterflies right back to the larger wingspans. Or not.
That's evolution. There is no *should* - there is only what is; and what is, is constantly changing. Bigger wings, smaller wings, it's all the same to me, until you can show me species *dieing out*, or having abnormally high rates of birth defects (and smaller wings are NOT a birth defect if they otherwise function normally), cancers, etc.
We should keep watching, with interest, what happens in the areas around Chernobyl and Fukushima, but so far, the evidence doesn't suggest catastrophic failure of life, nor is it likely too - the increase in background radiation was temporarily very high, but quickly subsided as the radiactive substances released by the plant dispersed and dilluted.
Finally, I will need a chance to look in more depth at this "study", but I have to wonder if they really proved these changes were due to Fukushima, and not due to something else which was co-temporal (e.g. result of selective pressures do to local ecological changes due to the tsunami; or possibly from the lots of chemical contamination of the environment due to the tsunami washing out industrial facilities, hospitals, etc).