I must the panic attacks from this are a bit overblown considering a couple of facts:
1) InstallCore has been deprecated for a while. If you try to google for it or it's SDK you will find absolutely nothing.
2)InstallCore itself wasn't a virus at all as many people think it was. It was actually an open source installation and content distribution SDK which is where it was classified as a virus because many malware creators used it to create infected installers for programs. Not to mention it was also free and widely used(100 million users according to wikipedia).
3) Ironsource shifted their focus towards mobile app development and monitization and their main area of focus nowdays are mobile apps , ads and games. They even own a mobile game publishing company called Supersonic studio. Many mobile developers use Ironsource for their mobile apps or games even the ones made in Unity.
Unity itself will not become a virus or become overloaded with ads. The reason why Unity merged with Ironsource to begin with was because Unity started to focus more on profit and mobile ads were earning unity a lot of profit and that's where Ironsource is hugely successful.
This merge will only massively change who the provider of mobile ads is going to be thou in reality nothing will change with this merge because you could already use the Ironsource SDK with Unity for a long time(since version 2017.2).
You can still use any older Unity version if you so desire. The only reason for change is because Unity as a company themselves shifted focus from the game engine to profits. The rest is honestly just fearmongering. While IronSource is shady rest assured that this will only affect the mobile game development and mobile ads.