Shippers are well aware that if they keep asking for lower rates they will run out of trucks ... but ... that's tomorrow, not today. When the time comes that they have no option but to pay, they will pay, but again, that's tomorrow, not today. Shippers are not responsible for your business, nor do they even care about your business. As long as someone will do it for cheap, shippers have leverage. Whether carriers allow shippers to use that leverage, or not, is up to carriers.
What carriers really need to understand is where they are in the grand scheme of things. Transportation is roughly one third of product cost (give or take a bit depending on product value) so when shipper go looking to decrease cost, they look at the biggest cost target ... transportation. Carriers need to be able to explain and defend their charges for no other reason than shippers typically have zero understanding of what it takes to run as a carrier.
Think about it this way ... Widgets are sold for $5.00 each. They only cost $0.50 to make. There are 1,000 widgets on a load. The load value is $5,000.00 and it only cost the shipper $500.00 to make them. Transportation is $1,900.00 to get them to market. The shipper is still making $2,600.00 on that load. Shippers make sure their freight is insured one way or another, so even if Super-Duper Lowball Carriers Inc. tanks the load, the shipper (under Carmack) still gets his $5,000.00 for the widgets. So, what is the shipper out by using bottom feeder carriers? ... absolutely nothing. In certain instances, like the market goes soft for widgets, it is actually advantageous for the shipper to use bottom feeder carriers in the hopes that they actually do tank the load ... it's a guaranteed sale. Properly used (abused), carrier insurance companies can be a shipper's best customer.
Of course that's an extreme example, and I know for a fact that it has happened, but the point is, it's a cold, cold world out there and you need to know how to dress for the weather.