the rules of accounting say losses are only recognized when realized (mostly because they don't want you playing the game in the other direction and exaggerating your worth), but the cashflows and market prices tell the true story, a different story than book value.
You don't buy bonds (or mortgage backed securities), you buy the future cashflows they represent. Let's say you have a billion dollars, and you buy future cashflows worth a billion at a 2% interest rate, then interest rates jump to 6%: you still own the same future cashflows, but those cashflows are not worth a billion any more, they're only worth a third of that (give or take). now you don't have the flexibility to sell those cashflows to finance a billion in other investment activities. Now your stock is not worth what it was, so if you sell stock you get less to finance investment opportunities. Will your bank be OK if you hold what you hold to maturity? you will get the cashflows, which if you have perfectly balanced cash flows-in with cash flows-out means you break even, but (a) you didn't do that and (b) that's not the business you are in, to sit in stasis for 20 years: you are much less valuable, your opportunities have dried up, and current customers do not want to do business with you any more. Sharks have to keep swimming or they suffocate (which may not be true for sharks, but it is true for banks) A bank is not a treasure chest waiting to be dug up, it's a McDonalds that needs people to order fries with that.
Roku put something like $500 million into the bank, that money gets invested by the bank overnight into very safe govt paper, but during the day it is turned back into cash assets of the bank, and it disappeared going into Roku's account and without being turned into govt paper again. It is not the case that Roku will get its $500 million back by sitting on that paper, there is no paper and the cash is gone.
You don't buy bonds (or mortgage backed securities), you buy the future cashflows they represent. Let's say you have a billion dollars, and you buy future cashflows worth a billion at a 2% interest rate, then interest rates jump to 6%: you still own the same future cashflows, but those cashflows are not worth a billion any more, they're only worth a third of that (give or take). now you don't have the flexibility to sell those cashflows to finance a billion in other investment activities. Now your stock is not worth what it was, so if you sell stock you get less to finance investment opportunities. Will your bank be OK if you hold what you hold to maturity? you will get the cashflows, which if you have perfectly balanced cash flows-in with cash flows-out means you break even, but (a) you didn't do that and (b) that's not the business you are in, to sit in stasis for 20 years: you are much less valuable, your opportunities have dried up, and current customers do not want to do business with you any more. Sharks have to keep swimming or they suffocate (which may not be true for sharks, but it is true for banks) A bank is not a treasure chest waiting to be dug up, it's a McDonalds that needs people to order fries with that.
Roku put something like $500 million into the bank, that money gets invested by the bank overnight into very safe govt paper, but during the day it is turned back into cash assets of the bank, and it disappeared going into Roku's account and without being turned into govt paper again. It is not the case that Roku will get its $500 million back by sitting on that paper, there is no paper and the cash is gone.