What other con? That wouldn't have similar issues with its staff?
Fishman doesn't pick OffKai because he's blissfully unaware of any issues. He's picking OffKai because they're the only game in town.
And keep in mind, while he pays OffKai, by financing the vendors, he indebts the vendors to him rather than to OffKai. This doesn't matter one bit while OffKai is the only game in town, but if a second con were to ever spring up and it becomes known that Sakana is jumping ship, then it'll matter.
As it is, in Fishman's calculus, the benefits of exposure and fan interactions at OffKai outweight the negatives. This may or may not be true, but it's clearly his calculus. And just in case an opportunity comes up, he's given vendors and artists a very good reason to jump ship if a second vtuber con were to appear.
Obviously, artists and vendors can attend both cons. But if one con pays for your shit and has Phase's fanbase attending and one does not, it rather affects the economic calculus.
The way out of this for OffKai is of course to become big enough to tank it. Get Hololive. And they're taking steps in that direction. They're getting a bit of Hololive this year. So, yeah, OffKai can defuse the bomb fishman has planted.
But Sakana's not cluelessly throwing money at a monster that hates him. He's very aware that the monster hates him but figures that the positives outweight the negatives. And planted an IED under the monster's bed just in case it becomes disposable.