You can undercut the mempool by offering frequent users not only a discount, but an opportunity to shed fee volatility. You can even undercut the mempool openly with transaction accelerators priced below market rate. (...) If you undercut the mempool market enough, you can begin to force many of the other miners out of profitability.
Why would that reduce profitability of other miners?
If some miners accept some transaction below market fee value, it means they incur opportunity cost on missing higher-value-fees transactions. And the opportunity cost of such selective miners becomes an extra profit for other miners (because they got more-than-expected higher-value-fees transactions).
If anything it should increase profitability of other miners.