Unlike most other treasury holders who can choose to cash out, stablecoins can't cash out as they must maintain their backing. They will need to continually roll over their treasuries.
In a very real sense, government debt expands the money supply, at least in terms of stablecoins. Of course, stablecoin growth is what triggers the purchase of treasuries, it's not the issuing of treasuries that triggers the creation of stablecoins.
With more treasuries going into stablecoins, there are less treasuries that banks can use as reserves to use for making loans. This can reduce the overall money supply from lending.
However, stablecoins enjoy two more days of economic activity than cash does. Thus, the velocity of stablecoin is slightly greater than the velocity of money. So the increased velocity could increase the overall money supply.
Whether the Fed raises or lowers rates, stablecoins still need to hold treasuries. Therefore, with the growth of stablecoins, the Fed's market activities will have less impact. There will be a market for treasuries whether banks lend or don't lend. It means there are fewer levers for the Fed to pull.
There are just too many factors for me to make any sense of the net benefit or net loss that comes from stablecoins.