Dedollarization means that the US market is far poorer and can't import as much as before, yet the new reserve currency(ies) make holders that much richer, enabling greater demand for either imports or self-consumption.
It will be an adjustment, but there will be many winners, none of which are the US. There may be some losers who do not negotiate trade deals or can't find new markets, but it's unlikely. The supposed US trade war on China has resulted in their GDP growing by a massive 5% last year. Canada, losing its exports to the US, negotiates deals with other countries.
The pain for other countries is more organizational than anything economic. The US will face massive economic disaster in the form of devalued dollars and the need to close the government deficit after decades of being addicted to massive reserve-currency-enabled deficit spending. The US will be plummeted into massive recession by those sudden changes, while the rest of the world merely trades with whoever are the winners.
If, for example, China decides that it finally wants a consumer economy, and the Renminbi becomes a partial reserve currency, the consumer demand will be absolutely massive. Europe may have a harder time signaling to Europeans that they are far wealthier, but make imports cheaper for Europeans, and people who find that they suddenly have a lot more left in their bank account at the end of the month usually find ways to spend at least part of it.
