Honestly (and it's just an opinion), it all boils down to one, possibly 2 factors.
The first factor, is strategy on Segas part. There's no doubt in my mind that the HD remasters exist. The rumors have been publically known that they are done and ready. They could have been announced anytime over the past 6 months, and my guess was that the plan was to have them released for this Christmas. Shenmue is a game set at Christmas so it's a no brainer.
But then we had the Shenmue 3 delay, and I'd say they want to release the HD remasters close to, but not too far away from Shenmue 3s launch date.
The debate there is that late 2018 isn't that far away, and that's very true, but then comes the second factor...
Deep Silver.
The deal was done over the summer, so the timing is very interesting. That's when the delay to 3 was announced, and let's be realistic about the delay, it's not set in stone that we will even see Shenmue 3 in 2018. We don't know the finances and resources (staff, technology etc.) Deep Silver will bring with their deal, but from what I learned at Gamescom, they are very on board with Yu-san and Shenmue 3. They have a great guy overseeing it all, who was not involved in the whole Mighty Number 9 fiasco. Add that to the whole "romanticism", element of releasing Shenmue 3, 20 years after the original release, and I would say that's my educated guess for that.
This, in turn, then sort of dictates Segas hand, and will domino effect the delay of a HD release. Also, to go one further, perhaps the HD remasters have been completed to a somewhat "satisfactory", level by the studio assigned by Sega to produce them, but Deep Silver have came along, taken a look at them, and may want to make them better than what they are. This could be anything, from something small like carrying over a save file, to something huge like overhauling the control system, or who knows, total remakes with the Shenmue 3 models etc. I dunno.
TLDR.
So to sum up, I'd say a HD release would be scheduled for around Christmas 2018, with a delay for a bigger, more exciting Shenmue 3, pushes to early 2019. Again, all opinion, and I could be wrong. But there ya go.
