Oh come off it. Opinion may be subjective, but when a considerable swathe of existing Shenmue fans have issues with, say, III's story (or massive lack thereof) that's a glaring issue. If it's not what a large chunk of its key demographic want, it's a problem.
These people aren't going to invest in IV so easily, if ever asked to. That's a glaring issue.
The meh review scores, while subjective, will scare away potential new buyers into the series and sales ain't looking good. That's a glaring issue.
You can split hairs about opinion on the game being subjective all you like, but when the future of the series rests in this community's collective enthusiasm / interest in IV, these opinions matter.