The block reward has been cut in half. So if they were mining for 0 transaction fees then their revenue would be cut in half, but nobody does that. The actual change will thus be less than half, and it's possible transaction fees will increase.

The event will not exactly come as a surprise for the miners: the date was more or less built-in the bitcoin algorithm from day one.

If it means the end of their business, it means their planning was fairly poor.