Saying that the fed should stretch out the timeline of asset sales so as to ensure they mature into their valuations doesn't align with how things work. Time value of money is a thing, if we consider even moderate inflation for instance, the cash you can get today is worth less in the future. In order words if SVB assets are worth $100 today, it makes no sense for me to hand them $100 in cash today while I wait a decade for their bonds to mature, just so I can sell them for $100 again.
That is before we get to the fact that the fed is not in the business of managing investments for random failed businesses.
If some knight in shinning armor takes a look at SVB assets and deem them worth some future value that would justify the current need for cash, they'll buy the bank. So far it appears no such knight has emerged.