Hear me out: I feel like a lot of people getting upset or chastising the fact that the new Korean vocal will be on V5 aren't looking at it from the perspective of the Korean fandom, who will ultimately be the vocal's target demographic.
Yes, it would be AWESOME to get him on SynthV! But remember, the vocal synth community is much smaller in Korea than it is in Japan or China or the West. SynthV hasn't really gained much interest there. Vocaloid's the only program that really took off there, outside of general interest in Utau. If they hope to have users, it's wisest to stick with what people know for now, yes?
"But V5's not as accessible as V3/4 was!" That's true. But being a Vocaloid will ensure more people hear of him, and if they like what they hear, it might generate a greater demand for high-quality Korean vocals on more accessible programs in the near future. That's the end-goal, right?
In short, we need to take a step back and remember that we're not the ones the new vocal is intended for. He'll be largely for a Korean audience. And putting him on Vocaloid, regardless of your personal feelings (trust me, I'd love to have him on SynthV too), is ultimately the smart thing to do. I think it's a respectable move and will be supporting him and his development!