Yakuza 1 opened at 130k. Black Panther 2 opened at 95k and Kenzan at 178k. All Famitsu numbers.
I think this series is pretty static all in all in its audience (kind of like MGS), with some highs and some lows. Citing Chris' numbers, here is the most recent release:
[PS4] Yakuza: Ishin! {2014.02.22} (¥8,600) - 73,086 / 129,536
[PS3] Yakuza: Ishin! {2014.02.22} (¥8,600) - 142,811 / 255,601
[PS3] Yakuza: Dead Souls {2011.06.09} (¥7,980) - 307,592 / 417,504
Seems pretty consistent if anything,though we'll open lower than Dead Souls. A remake of a 2005 game is fairly close to a new release game or on the level of a spin-off, so I think saying "oh its just a remake" is a bit superfluous of a term in this case since its not just a remaster but a remake (unless I am mistaken).
The audience has simply moved from PS3 to PS4, but it doesn't seem to be growing.