I have never heard someone complain about a good classic pixelart game as looking bad. Nobody looks at pic and says this looks "looks old". Great pixel art will ALWAYS hold up.

Now, do people look at Deus Ex or Morrowind and say it looks old? Absolutely. This is not the fault of developers though. At the time, 3D art was a young medium and artists had not had time to develop it, whereas Metal Slug came long after pixel art was a mature genre. And look at 3D art, just a few years later we got great cel-shaded works like Wind Waker and TF2.