Yu was one of the biggest game designers in the world during the late 90s and the early 2000s. He was in a top management position of a four-thousand employees corporation and basically had a blank cheque to a billionaire's bank account (Isao Okawa).
Now, Yu is a self-employed guy with a small project company of a couple dozen people, who hasn't finished a non-mobile game in almost 10 years. Sega Race TV being the last one I can think of (and yes, the game has the word "Sega" in its name, I think it was during his time at Digital Rex, while Yu was still at Sega). Also, if you look at his track record of the last 10 years, a bunch of his projects got cancelled, like Psi-Phi and Shenmue Online. Both are not his fault in my opinion but still, they're on his resume.
Also by now, YS Net probably depend on Deep Silver's money to even be able to finish S3. And they're under extreme pressure time-wise. The PS5 will probably come out at some point in 2020 and Sony building up the hype for it in the months prior to its release could leave the PS4 dead in the water. If they miss the PS4's life cycle, they might as well redirect the entire project to the PS5 but I don't see that happening. Who's gonna foot the bill for that? Missing the PS4 would in all likelyhood get S3 cancelled alltogether (at least the console version of it). They have to get this game out of the gate in 2019, early 2020 at the very latest, if Deep Silver is willing to fund yet another delay, that is. This time, there's not gonna be any pushing it back further.
So given all the factors I mentioned, with Yu's situation being nothing like 15 or 20 years ago, I think it's only natural for people to be a bit wary of Yu starting another project on the side. We're 10 months away from the deadline given to Yu by Deep Silver's CEO.