How do we make sure we don't lose some data permanently?

[Bitcoin]

How do we make sure we don't lose some data permanently?

If my understanding is correct, the blocks, transactions and other data are stored in a decentralized way. Even though the full nodes exist they may not have the complete data and it may be fail/removed at any point of time.

So, how does the system ensures that we don't lose some important data. I know that the probability of all the nodes containing a transaction or block going down is very less. But is this the only reason the data is safe? Is there any math supporting the claim and showing that the probability is extremely low? Or are there any other rules that ensure this that I am missing?

Please correct me if my understanding is wrong. I just started studying about blockchain.

Source: How do we make sure we don't lose some data permanently?