Risk undermining American leadership?
That already happened. America has significantly diminished standing on the world stage. Leadership requires strength, stability and honour. Leadership is about inspiring others to follow. Trump is seen on the world stage as a problem. His strength, as projected to the rest of the world, is based solely on the machinery and heritage behind him that he is seeking to dismantle. Ironically he said to Zelenskyy, "I made your a tough guy". Trump is only a tough guy because the US military stands behind him. Stability and honour are clearly gone. The US government won't even honour it's own domestic contracts with US citizens and organisations, tariffs or on them off and the stock market is unstable as a result.
Any government dealing with the United States currently has to weigh up whether the deals made will be honoured. Many of them will choose to believe, based on the credit America has built up over centuries, that they will. But we all know that Trump has no qualms about cancelling a contract he decides he doesn't like.
Over time the EU will move to be self sufficient. America will find it's influence and GDP diminished as a consequence.
You cannot agree to buy fighter jets from a country that may threaten to change the deal part way through, or remove support. You cannot make trade deals with a country that might decide in 2 years that they didn't negotiate as well as they could and refuse to hold up their end.
The only reason anyone is still listening to America right now is because the memory of what they used to represent is still so fresh. There is hope that even though Trump says and does one thing, it is just rhetoric and the ocean taker that is the US government cannot be turned so quickly. These hopes assume due progress would be followed.