there is a fundamental misconception in your analysis that miners dump the coins they mine on exchanges, effectively putting pressure on the price.
in reality, there is a high probability that most of the mined coins, if sold, are sold in bulk to brokers OFF-exchange. there are numerous brokers out there offering to buy in bulk at a premium and considering privacy issues and exchange slippage it only makes sense for miners to get into supply agreements with them.
Close, but in reality miners sell at a discount to OTC resellers, in cash, to avoid taxes. This is true for many of the Chinese miners.
Those OTC resellers turn around, and sell OTC at a premium to clients that want to avoid KYC compliance (no Coinbase, no exchanges).