From what I read, fans in Japan were very mixed on 6.
A lot were torn on the ending (I kinda was too), a fraction of the content that Yakuza 0 provided, and had poor performance. So this led to some pretty poor word of mouth traction after the initial release. It wasn't until Kiwami 2 from what i've seen that fans were universally back on board with the series. Most people chuck Kiwami 2's sales as it not being a mainline entry and being the first solo console entry of the series since 5. So it'll be interesting to see how well Shin Yakuza performs now.
Admittedly I do have mix feelings on Yakuza 6 as well, but I thought the decline began before 6. But yeah it will be interesting to see how Shin does.