We can see the current Ethereum size is greater than 1509GB, and Ethereum's gas fee is too expensive. My question is what happens to fees when Cardano's chain reaches 1509GB data?
The fees are based on the size of the transactions and have nothing to do with the blockchain size. I think the same applies to Etherum.
About when we will see the size of the blockchain depends on the usage of network of course. But if we keep the current
max_block_size set to 65536 bytes and account for about average of 4320 blocks a a day it is about 280MB a day with full capacity usage.
In theory, it would take about 14 months to reach 1 509 GB of size if the network has been utilized to its fullest potential, almost 11 years if we use only 10% of the network capacity.
But that is very theoretical and it the real world, we will probably get shredding mechanisms before reaching that size.
The transaction fee for Ethereum does not rely on the size of the blockchain, but how many transactions there are in a given time. Like I understand it Ethereum uses an auction model for transaction fees, meaning that more transactions (more demand) equals higher fees.
Cardano's fees are constant and do not rely on either the size of the blockchain or the transaction demand, but can be calculated like this:
Transaction fees (in ADA) = 0.155381 + 0.000043946 x [size of transaction, in bytes]
The average transaction fees is currently at ~0.22 ADA per transaction (more details here).