The storage costs on bitcoin

Parts of the blockchain from the past get rewritten as transactions ‘complete’. 

If folks are using bitcoin fr Christmas spending at Walmart, The transaction path will end up agglomerated at WalMart addresses. Some blocks with a lot of recent Walmart activity may get re-written, say after the winter sale, when all the money is collected. This is the part about reducing empty space in existing blocks by repacking valid blocks, it comes under the moniker orphan block in the literature. The effort of repacking old blocks goes down exponentially as transactions complete, the upper blocks become packed. The repacking cost is storage cost and is small, and repacking reduces look up times later.

The problem is apparent when the guy at the tip of the chain ges n a spending spree.    In that case, the spender generates a larger proportion of transactions, and the transactions will cause considerable re-write as they arrive because there is a packing of his transactions that is optimal, and will constantly be computed.