It's not difficult at all. This is the real world; physics is not paralyzed just because mathematical definitions don't fit well.Dave, is it not difficult to maintain scientific veracity without mathematical rigour?
Remember the context here:
For a photon to have zero frequency, it would need to have a wavelength infinitely long. But a photon whose wavelength is longer than the diameter of the universe is effectively infinite. To-wit: being 100 billion ly long and being infinitely long makes no change in it the property of its frequency; it's still undefined.
So: a wavelength greater than the diameter of the universe is "as close to being infinity as is meaningful".