Percentage of local Japanese passengers in/out of HND using bus to access HND seems to be low compare with monorail and train. My usual destination is Setagaya and most common reason I hear from local Japanese not using Limousine Bus to HND is the cost. From Tokorozawa the bus to HND is 1540 yen. Although it requires two transfers, train from Tokorozawa-(Seibu)-Takadanobaba-(JR)-Shinagawa-(Keikyu)-Haneda Airport is 1050 yen. From Musashi Kosugi the bus to HND is 920 yen, where on train requires two transfer which includes little walk at Kamata but Musashi Kosugi-(Tokyu)-Tamagawa-(Tokyu)-Kamata-(Keikyu)-Haneda Airport is 540 yen.

My usual destination is Shinjuku and train on Haneda Airport-(Keikyu)-Shinagawa-(JR)-Shinjuku is 610 yen where Limousine Bus is 1230 yen. When I took Limousine Bus from HND to Shinjuku were when I had large luggage and every time there were only few local Japanese passengers and most of passengers were non-Japanese. When I do not have large luggage I always take train to Haneda, and local Japanese say same thing. Local Japanese I talked with say they will use Limousine Bus when they have large luggage but that is not most of the time. Domestic passengers are still large portion of traffic at HND and many domestic passengers do not carry that large luggage and many choose train/monorail over bus simply because of the price.

If JR actually make Haneda line I highly doubt trains which JR will run to Haneda Airport will be NEx or Skyliner type train, but will be more like commuter train.

If your travel out of HND is primary international with large luggage then Limousine Bus or NEx or Skyliner type trains will be convenient, but for big part of passengers using HND they choose cheaper option.