How much does it cost to store a full block?
Maximum Block Size = 51200 bytes or about 160 GB per year.
You are right, we will have to increase transaction fees to cover storage / bandwidth costs if we end up maxing out every single block.
If the average transaction is 1KB in size, that means we have about 50 trx per block or $1 per block or $31,000 per year in transaction fees per delegate to cover the costs.
If someone posts transactions that are 50KB each revenue would fall to just $624 per year per delegate. Or $624 per year to store 1.6 GB of data, much cheaper to use free services rather that spam blockchain.
That is a fairly large discrepancy in revenue, but I suspect increasing the fees slightly if this was abused.