It would be both embarrassing and damaging to our brand if we were to build a stadium on the docks that's even smaller than Anfield. It would just confirm our status as the city's inferior club. No one's saying we should try to put Old Trafford in the shade, but if it's going to be smaller than Anfield and smaller than Spurs' new stadium (61,000 all-seater), then we might as well not bother.
I know that some are concerned that we would be unable to fill a stadium with a capacity of over 60,000 (not that failing to fill our new stadium would be detrimental to us in any way), but our highest ever attendance is close to 80,000. And a move to a new stadium almost always increases attendances massively. Arsenal could never fill Highbury, which had a capacity of just 38,000. Conversely, the Emirates, a 60,000 all-seater, is always packed.