That's the problem. If YS Net keep dragging development on and on like that, chances are they might as well move the development to PS5. If S3 comes out after the PS5's launch, a lot of people will have lost interest in PS4 games by then. Let alone the fact that the remasters are coming out on PS4 which would mean that if PS5 doesn't have backwards compatibility, the first two games would again not be playable on (then) current-gen consoles.
They need to get S3 out on PS4 and in 2019, otherwise Shenmue will probably be in deep trouble again. With the remasters coming to PS4, there's no way YS Net can afford to let this console generation pass.
Looking at the life cycles of Sony's past consoles, they each had a life span of 6 to 7 years.
PS1: 1994 - 2000
PS2: 2000 - 2006
PS3: 2006 - 2013
PS5 will probably come out in 2019 or 2020. I really hope the stretch goal talk is pure marketing this late into the game's development and a way to generate some extra revenue from the slacker backer campaign. If Yu is really thinking about adding additional features to the battle system as late as now, he might wanna think twice. This game needs to come out in 2019. One of the most crucial factors for the long-term future of the series is to avoid for S3 to come late to the PS4's party.
Bringing out S3 on a last-gen console which PS4 will become sooner or later, or on a new console with no user base installed yet both don't seem to be wise moves to me. Let alone the costs of moving the project over to PS5.
With licensing out Shenmue to Yu and bringing the S1 & 2 remasters, Sega have done their part. More than could have been expected of them to be honest. Now it's time for YS Net to do their part of the job. And they'd better bloody well deliver! We as the hardcore part of the fan base may be happy to wait for S3 to come out as a PS4 game in 2020 or 2021 but chances are the rest of the market will not..
I'd estimate Yu's got another year, one and a half years max to deliver a great Shenmue 3. If YS Net can't pull that off, who knows what will happen? Let's not kid ourselves, here. This is probably the last chance Sega, Deep Silver or any other publisher for that matter will give Shenmue as a franchise.