I mean, this is totally reversible and is mostly caused by hitting the zero lower bound. Consider that money is put into circulation by the fed buying safe assets. They can sell these assets to reverse this.
Not that there isn't any danger. Even better would have been if the Fed was ok going into negative rates (like the ECB did a few years ago). They wouldn't have had to create so much money. People, businesses and banks are just hording government money instead of private assets because it offers above market returns of 0% while marginal safe assets on the private markets have had negative returns (on a risk adjusted, liquidity adjusted basis). With sufficiently negative Fed rates more in line with the markets, people would have kept their assets instead of hoarding government paper and the Fed wouldn't have needed to print so much.
Not that there isn't any danger. Even better would have been if the Fed was ok going into negative rates (like the ECB did a few years ago). They wouldn't have had to create so much money. People, businesses and banks are just hording government money instead of private assets because it offers above market returns of 0% while marginal safe assets on the private markets have had negative returns (on a risk adjusted, liquidity adjusted basis). With sufficiently negative Fed rates more in line with the markets, people would have kept their assets instead of hoarding government paper and the Fed wouldn't have needed to print so much.