OK Spath simple Laws of Supply and Demand
If the price of something goes up, less is demanded of that product. Trips to the EU countries are a product and they are competing with other non EU countries (the Americas, Africa, S E Asia, even Switzerland and Norway for your tourist cash. Less people will go on holiday to the EU countries - tourism will not stop, but it will reduce.
The EU will have to do the sums and work out whether the extra charge is worth losing the tourist revenue.
This is in addition to whatever way Britain decides to act against EU travellers.