My concern is that the lack of attention will lead to low sales and we'll be waiting another 20 years for a sequel, if one ever was to come.
I hoped Shenmue III would just wrap up the story. Can't deal with the stress again.
Whilst I understand where you are coming from, I think you are only looking at all of this from one side of the spectrum, and unfortunately it's the negative half of it all.
Let's start off at the beginning and look at why it's been so long to get Shenmue 3 going. There were so many hurdles to jump over, but all in all, we cleared every single one to get to where we are now, and oh boy did it pay off. First we had to get Yu-san to commit to it, which he did. Sega had to relinquish the rights to their IP for him to make it. Did that. Next up was the funding, and with Sony's help, not only did it get the funding, but it got it so organically. It got the love, not just from loyal fans, but also attracted a major publisher, in the form of Deep Silver, to also come on board for the ride. We know Shibuya also has a hand in the pot, and no doubt Sony will help with the marketing of this game. They will not desert that, as they know it would be a bad PR move. That's all Sony has left to do. They already went above and beyond what they needed to, by giving stage time to announce a Kickstarter on their biggest stage of the year. No one should undermine that gesture, and what did for the success of that Kickstarter awareness.
In regards to the future, and specifically the sales figures of Shenmue 3 to Shenmue 4 development.... You have to realise that this game is already paid for. Like any other business, money is put into creating something up front, as an investment, with the hopes that the money is recuperated, and then continued to roll in to make a profit. Shenmue 3 doesn't have a defecit to pay back. We paid for it, so almost every penny spent across the world on this game is profit, and no doubt will be invested into Shenmue 4. It's essentially free money for YSNET.
Now, if this game sold a modest 100,000 copies at £40 a pop, then that's 4 million on the bank. But hang on, that's not enough to make another game no? Whilst it may seem small, take into consideration that another piece of the hard work is already done; the game engine. That's already done, with the help of Unreal 4. The staff are in place who now have 3 years of working day on and day out with each other. Learning the software. Creating assets. All that stuff is done, and if the worst possible outcome is that Shenmue 4 was made reusing these assets Majora's Mask style, I doubt anyone would be complaining, considering the position we have been in for almost 2 decades.
Ultimately, in terms of another 20 years wait, you really don't need to have that concern. Not at all. Shenmue will see a conclusion to its story, I have zero doubts about that.