It's not zero cost, but you might pay half a percent or a third of a percent, especially in major corridors. It's quite cheap, though not free. In a stablecoin world, does the infrastructure they've built to offer such services become instantly available to every other fintech, allowing them all to offer similarly low costs? Does that diminish the advantage that Wise or Revolut currently have?

I would strongly recommend reading the very recent annual economic report, chapter three, issued by the Bank for International Settlements. In this report, they argue strongly against stablecoins. The paper outlines three main reasons why stablecoins are deemed insufficient as money or as an alternative to the current dominant monetary system.

It's not zero cost, but you might pay half a percent or a third of a percent, especially in major corridors. It's quite cheap, though not free. In a stablecoin world, does the infrastructure they've built to offer such services become instantly available to every other fintech, allowing them all to offer similarly low costs? Does that diminish the advantage that Wise or Revolut currently have?

First, for widespread use, money must ensure system integrity, including compliance with anti-money laundering expectations. Second, there must be singleness of money, meaning one unit of commercial bank money should equal the same amount in every commercial bank for the same currency. It should also be freely exchangeable for the same value with cash or other forms of central bank money.