Actually telcos in those countries with many foreign visitors will still make money. While consumers enjoy 'roam like at home' without additional costs, there are still wholesale roaming charges in place for the home carrier [0]. A quick back-of-the-envelope calculation shows that these wholesale roaming caps are comparable to typical consumer prices, meaning they should result in actual profits (at least compared to German prices, which are generally not the cheapest).
[0] https://en.wikipedia.org/wiki/European_Union_roaming_regulat...