So a miner will never reduce the cost to include a transaction into their block, even when they aren't getting enough to fill up the block?
>They might stop mining altogether
Maybe, but any market can experience short term irrationality. Maybe it takes them a few hours to stop mining in which they lose money. Or maybe stopping operations costs enough money that the miner won't stop even at a small loss, at least for some amount of time.
Sure, if you want to twist the wording like that, they "lower the fee" to the point where they can fill the blocks. But they're not really "lowering their fees". They don't even have a concept of the fee level they're "charging". They can either accept the fees that are available or not.
If they are mining at all then they want to accept the best fees that are available. There is literally no rationale for them to be mining non-full blocks when fee-paying transactions are available to put in the blocks. It's not like it costs more to mine a larger block. The cost to mine a block is fixed, so you may as well get as much fees as you can find.
Couldn't the same be said of a brick and mortar store? They either accept the offers they are given or they don't. That for some item they only accept offers of exactly 9.99 (plus tax), rejecting not only lower offers but higher offers, doesn't change that the interaction can be described in the same fashion.
All the rest also applies to normal supply and demand. When you do a production run of some item, the cost tends to be fixed per item. Doing another run at a different time may cost different, and it is possible for something extreme to happen (factory accident), but in general the cost of production of a single run is the same.
I see nothing about this that would void basic economic reasoning, where things like 'reducing fees' happens in certain conditions.