Releasing afterward, during the holiday season on PC digital marketplaces, would be asking for disaster, whereas some will at least look at Ys Seven if they're already into the PC Ys games and get it later if they have to. VIII's impending release will bring more attention to Seven and perhaps not cannibalize sales too hard if XSEED takes advantage of these upcoming weeks to promote the port.
The problem with VIII, more than being new, is that it's likely good/great enough to turn people off from trying Celceta given that game's bad word-of-mouth. XSEED might just skip it altogether unless they can find a way to deal with lower sales (like, IDK, better social media presence/reminders).