Of course it depends on what happens, but I feel the reason olympia fell to 220 was less the fact it was their second CEO to fall, but rather the way she fell. We have no idea how much it fell when Syndrome died going from ??? to 310, what caused the drop by 90 was likely not Evelyn getting removed but her getting arrested and the entire supervillain plot unraveling. Shareholders, Executives, and regular employees were all jumping ship to try and avoid arrest and deal with the trauma they went through. Di Amara is, well, being just as evil. I also imagine if we infiltrate the company it will be easier to take over.
The Zaibatsu is a bunch of smaller companies rather then an entire kingdom in and of themselves.