First of all, roaming charges is only about two percent of their revenues and since people will definitely use their phones more when abroad they'll probably gain most, if not all of that back.
Second of all, the death of roaming charges means companies will be able to increase their customer base pretty easily. Plus, it will give people living close to borders, or people living in one country and working in another, a lot more options.
The French operator Free offers free minutes, free texts and 3 GB of data for EUR 20 per month. Now, imagine if they'd include free minutes and texts to Belgian, Luxemburgian, German, Swiss, Italian and Spanish cell phones (they already offer free minutes to all European fixed lines). Can you imagine how many more people would be potential customers to them just by doing that?