I guess it's one less cash grab to wring some money out of their old engine.

As long as it comes for PC I'm fine with it.

Obviously it's so that six months later they can release Assassin's Creed: Rogue The Definitive Edition on PS4/XBO.

It's not, PC is 8th gen and it gets both

Developers have been doing this forever... for example at the beginning of last gen the 360 got a port of COD2 alongside the PC version, while previous gen consoles got COD:Big Red One, a spinoff of the storyline of COD2.

so it doesn't take sales from Unity on current gen. And then they can get double dips when they do a definitive version next year.

Because if Unity doesn't cover its costs with such a small install base, the previous gen versions will cover the costs for it :D

To re-release it next year as the Assassin's Creed: Rogue Remastered that will include all DLC except the DLC they release the day after re-release.

Because it will sell more on last gen consoles.

But as some people have said there will probably be a remastered version in a years time.