I don't see how an HD port really achieves that. It's going to bomb, and will provide no justification to bring 6 to the Wii U. It would make more sense to port the games over to the 360 and release them globally or something.
Hey, I didn't say it would work. But they've tried the global release strategy - multiple times. It doesn't work. The game is too Japanese-centric to appeal to anything more than a tiny niche in the west. They would have to really beef up the production standards (even higher dev costs) to really make a push in the west, and they would need to change the focus of the games to give it more appeal to the west (which would run the risk of diminishing the sales in Japan).
Anyway, it's a strategy they've tried, and they seem to have realized that the sales in the west are pretty much cancelled out by the localization and global publishing costs. I wouldn't be surprised to see more Yakuza games find their way westward, but that will just be Sega dipping their toe in the pond occasionally. It's not a strategy to increase the viability of the brand moving into a new generation.
So with the west out of play, what other options are there? As I said before, I can't imagine that Sega will want to keep the series going for another full generation on the PS4 if the sales for each title are ~500k. So they need to find another avenue for growth. They can try reaching out and building up a new fanbase amongst the Nintendo fans. Or they can try going handheld (3DS and/or Vita), but that doesn't seem to be the direction that Nagoshi wants to go.
It's all just speculation - but I mentioned back a number of months ago that I suspected that we'd be seeing Yakuza find it's way to the WiiU - for the very reasons that I just laid out. The series just doesn't sell enough as it is for Sega to justify another bump in costs without another source of additional revenue. Maybe they try the HD 1+2 thing and it bombs, and they reevaluate. Maybe it's the first step in a process to try to slowly build a fanbase so that an eventual PS4/WiiU multiplat strategy can exist. But they need to grow the audience for the game, and there aren't really any good options for doing so.