While Kiryu's one true love, best Yakuza gal Kaoru getting written out of the story will always smart, I wouldn't say Yakuza 2 is any less relevant than any other game. It introduced Daigo and set him up as sixth Chairman, introduced the Osaka Yakuza and gave us Sotenbori, Ryuji was in a couple of Yakuza 0 sub stories, and, yeah, there's a little reference in 6. Yakuza isn't really an 'overarching storyline' kind of series.
And, yeah, Kiwami 2 has all the makings of an incredible game. The original was my favourite in the series until 0 came along, which edged it out with all the advances in gameplay, storytelling and design the team had learnt over the years, but Yakuza 2 on the Dragon Engine with Cabaret Clubs and the new Majima/Makoto story chapter should be brilliant.