Countries want to hold USD because this nation is seen as safe, strong and economically vibrant. The US has never defaulted on a payment and the consumer base is strong. However, the use of the USD is also because there are no better alternatives. If and once a better alternative comes out, then there is no business case for a foreign nation to hold USD anymore. That money will come back if countries cut their holdings dramatically.
That money is held in bonds. It's not like foreign nations are just gonna empty their piggy bank and buy shyt in our economy artificially increasing the money supply.
There implications if the Yen becomes the reserve currency, but it's not going to be an influx of US dollars in this country resulting in inflation.