So why doesn't Ethereum just reduce the DAG file size? Just clip off the first 2GB of the file and go from there? 3GB and 4GB cards instantly become usable again, nice and simple.
The ETH devs goal is not to benefit miners, they actually don't like miners with all the miner fee rewards reductions they implemented in the past. Their main goal is to get out POS as soon as possible.
Why did ETC do a DAG reduction? They basically had no choice because their chain was being 51% attacked like half a dozen times. So they reduced the DAG so old GPUs like the 3GB and 4GB could mine again for a little profit and the increase hashrate would prevent any 51% attacks.
The ETH blockchain is very secure with all that hashrate, doesn't make sense to reduce the DAG.