Mining cryptocurrencies involves a number of risks. One of them is that mining difficulty increases every day, which negatively affects ROI of any mining gear you own. Only a psychic can tell in advance how profitable a GPU or ASIC will be in, say, a month. In addition to that, cryptocurrency value fluctuates a lot. For instance, the past month Ethereum price almost halved, which made many people ponder whether they should switch to a different cryptocurrency.
Over time, this file increases in size, which puts additional stress on the GPUs. Once the file reaches certain size, some GPUs are no longer able to hold it within their VRAM, which renders those completely unusable for Ethereum mining.
It looks like Polaris-based GPUs have some architectural issues that make them lose Ethereum hashrate in proportion to the DAG file size. The bigger the DAG file becomes, the slower Polaris GPUs mine.
In my first post about the issue, little was known about the causes, and I honestly believed that we are dealing with an architectural problem, rather than a software one.