What territory are you referring to here?
PSV Ys VIII: Lacrimosa of Dana 47.681 78.070 Nihon Falcom 21/07/2016
PS4 Ys VIII: Lacrimosa of Dana 30.764 63.936 Nihon Falcom 25/05/2017
SWI Ys VIII: Lacrimosa of Dana 6.817 9.101 Nihon Falcom 28/06/2018
If it's Japan then I'd have to ask, what is the declining market? Despite being a "late port" Ys VIII for PS4 sold very very well compared to what it sold originally for Vita. Then the Switch came out a year after that and sold much, much less. So I'm not exactly sure what you're referring to as a declining market.
Edited to remove digital numbers from PS4/Vita.