First, the scarcity (deflationary nature) of something does not determine its monetary value. It is desire and need vs. ability to access that creates monetary value. If scarcity created value then an old pair of your shoes would increase in value over time as there is only one pair of your old shoes.
The value of bitcoins are determined by desire and need along with the ease it is to acquire them. If borrowing bitcoin was easy, the monetary value of bitcoin would go down if you just look at the scarcity component. However, if the ability to borrow bitcoin caused the desire for it to go up, that could cancel out the negative monetary affect of the increased ease of access. It is not a single factor that causes monetary value.
Second, your comments about banks is not true. Banks do have collateral for loans, that is correct.
However, many, many times, that collateral does not make up for the value lost when a loan goes bad. There are millions of examples of this. Credit card debt is an easy one to understand. Everyone has likely known people who racked up too much credit card debt and then went bankrupt. The banks lost the money they lent in that case.
When credit card debt goes bad, banks usually sell the bad debt to a collection agency at a loss so they don't have to deal with it. You can buy collection agency debt yourself if you want and try to make money off it. There are many collection agencies. Some do decent business, some don't. Similar to banks you can simply type "collection agency for sale" in a search engine to find one if you want to buy one. Buy up the debt and hire a bunch of people to start calling people to try and collect. The banks certainly do not make a profit selling their bad debt to these agencies. The really bad debt can be bought for pennies on the dollar.
The reason the financial crisis in 2008 was so bad is because banks were making 90% or 100% loans (or more) on homes at their then market value. Housing prices dropped 50% so people didn't pay back their loans and the banks ended up with huge portfolios full of crappy homes that they couldn't manage and needed to sell. The government had to step in and loaned money to the banks so they didn't go under. And the government purchased many of the banks crappy loan portfolios. And they merged together banks with terrible loan portfolios with stronger banks. And the work done did keep the system from failing. The banks lost real money and many are still today digging out of that hole. It is the main reason the Fed has been keeping interest rate low.
So, no, banks don't always have a way of recouping the money they lose. f it was that easy, we'd all own banks and the richest people in the US would be bank owners. If you look at the rich list, they aren't the owners of banks. They are tech company founders, Walmart heirs, hedge fund traders and the like.