The OP is hilarious, like you can do a single thing to stop them! Everything will be chaos? The fuck are you talking about? You act like Disney come in and destroy everything, like Hasbro doesn't do that themselves already!
Disney has actually done very well with the companies they have bought, they tend to either keep people in the right positions or replace them with better people and they do leave them to their own devices (Marvel & Pixar for example). If something is working in a company than Disney leaves it alone. If a company like Hasbro does appear to be on sale, then I can think of far worse companies than Disney going by their acquisition background.
The only thing Disney does mismanage is it's own stuff.
As for why? Why not? If I had the money I'd try to buy many companies.