The actual agreement was done a few years ago, part of which was the "gradual reduction of roaming costs over two years".
Your point about Three is actually a reason why this agreement was needed. Why have none of the other carriers done the same? They have had years to act on this or make some effort to reduce the costs of roaming, yet remained complacent.
Don't assume the actions of one carrier represents all of them. Three is a great carrier but their market share is small compared to others. Other networks likely didn't make any efforts to reduce the costs for customers because they didn't need to. Additionally, Three's motive may be more marketing and attracting new users than simply for the good of their customers.
Your point about Three is actually a reason why this agreement was needed. Why have none of the other carriers done the same? They have had years to act on this or make some effort to reduce the costs of roaming, yet remained complacent.
Don't assume the actions of one carrier represents all of them. Three is a great carrier but their market share is small compared to others. Other networks likely didn't make any efforts to reduce the costs for customers because they didn't need to. Additionally, Three's motive may be more marketing and attracting new users than simply for the good of their customers.