This would only be the case if the RMBS consists of whole mortgages. My understanding is that each mortgage was split into various income streams, with a stream getting the first x% of dollars obtained either through payments or foreclosure, one getting the next y% of dollars, and so on. Therefore, if your tranche only gets the last 10% of the repayment of the mortgage, getting less than 90% through a default where the property resale price + payments made falls more than 10% short means this tranche is worthless.

In your example, the sale of that RMBS at 70 cents on the dollar should have forced an immediate markdown of assets to show the worth of similar tranches at that value, but they are not forced to sell because of this. If the markdown forces them to sell assets to meet margin calls, there are sovereign wealth funds and other investors with money that would be willing to buy something below what it is worth so they should be able to sell if for market price which would be 70 cents or higher.

The fundamental issue is that these banks are considering tranches to be level 3 assets that do not have a market price so they can claim they are worth whatever their models tell them, whether their model is close to reality or not. These institution's models are so far from reality due to faulty premises that even a single sale considered to be non-distressed of these assets would require writedowns that would wipe out all of their capital. Their inability to sell these securities is in reality a refusal to accept the actual worth of these products, so they want their rich uncle to pay an inflated price for them.