Why does the Fed feel the need to reduce its balance sheet? Why does the Fed feel the need to reduce its balance sheet?

What is the problem with the Fed having a large balance sheet long term? What would happen if the value on the Fed's balance sheet collapsed, say in a bond crash?
This is veering into the direction of an opinion-based answer, but I would argue that there is no particular evidence that the Fed “needs” to reduce the size of its balance sheet. 


If the Fed wanted to revert to its pre-2008 mode of operations - where interest is not paid on reserves - it would be required to reduce its balance sheet so that there are no excess reserves in the system, if it wanted the Fed Funds rate to be greater than zero. In the aftermath of the Financial Crisis, many commentators  assumed that this was the case. However, the pre-2008 Fed operating procedure was unusual relative to what other central banks do, and there is no “need” to revert to it. By paying interest on reserves, it can place the overnight rate wherever it wishes (with small spreads).
The Fed does not mark-to-market its portfolio. Market value losses on bonds does not present an issue, as they can hold to maturity. The only “risk” is that the cost of reserves is higher than coupons received. However, like all central banks, it can operate with negative equity. At most, negative equity is a political embarrassment.
Are there economic effects of a large balance sheet that need to be undone? This is a disputed area of research. My opinion is that the side arguing that the size of the balance sheet matters have not presented strong evidence of this claim. One would need to look at claims about the effects of the balance sheet growth, then see whether they hold up. From what I saw of the post-Crisis literature, the effects were not clear-cut.