I remember from elementary econ that monopolies are supposed to be bad because they can cause economic distortions that cause the usual supply and demand reactions to fail; and to fail in ways that increase the power of the monopoly. From history I note that large concentrations of money have a tendency to corrupt politics and still further concentrate money and power at everyone else's expense. So far, so bad, but I haven't heard warnings about the effects of failure. Maybe I just haven't read the best econ texts, but...

Sooner or later, every company will fail. Companies are run by people. The Peter Principle applies. People screw up. Men "drunk with sight of power" fail to take advice.

If the company is primitive (dig ore, haul to market by oxcart), it isn't too hard for somebody else to pick up the pieces, but when a complex firm bails, it can take a lot of time to glue it back together. Teams break up, shops get sold off, notes come due with nothing to pay them with...

If said company is vital: a monopoly of something essential, or carrying so much debt and so many orders that it will take down a large fraction of the other firms in the land when it fails, then your economy is in deep danger. Korea had a few firms that were "too big to fail," except they did. I hear Japan is in the same boat.

So, for economic safety, you need to discourage large monopolies, and worry about mega-mergers. As well as the other reasons I used to read about.