I suppose it depends where one wants to go. The most difficult pieces in the sheet music list are all indicated as being at grade 8+ level, whether they are by Bach or Liszt or Rachmaninoff or Debussy. Still, I have a feeling Bach would have found it almost impossible to play a Debussy etude at sight (I actually can't be sure, but that's my guess).
Now considering that in Bach's time, nobody had even dreamed of the romantic or modern period's music, and were nevertheless perfectly happy and didn't feel they were missing out on anything, I guess one could argue that if you like everything up to the baroque but not much beyond, that it is perfectly legitimate to specialize in that kind of music and never bother with anything else. I suspect Bach's more difficult pieces will also equip one with a very substantial part of the technical requirements to play almost anything else.
Still, if you stick to Bach and his contemporaries to the exclusion of everyone else, and then one day decide you want to tackle Liszt's B minor sonata or Debussy's "Reflections on the water," you may well find that you need an extensive period of adjustment.
That, at least, is my guess. At this point my own technique doesn't extend much further than "Für Elise," so I am kind of speculating here... :-)
Topic: Music of Bach, is that enough?