If you have a monopoly, they can arbitrarily raise their prices without increasing the speed at all. From the situation you described in another post, it sounds as if they were going to raise your bill no matter what, and pushing you to a higher speed tier was just the rationale they gave. At some point it becomes more expensive for them to maintain old equipment than to replace it, and if the replacement equipment has much higher rated speeds, they're not saving anything by continuing the lower speed plan. To the contrary, a lower bill just means a lower profit margin.
Consider: Let's pretend that broadband speed worked like a typical commodity so that the producer cost for 10Mbps was roughly ~10x as 1Mbps. And let's say your bill for 10Mbps was $30 (producer cost $20). Would they offer a 1Mbps plan for $3 (producer cost $2)? No, because they would be cannibalizing their own customers. They don't want $1/month profit per customer, they want $10+/month profit customers. And it real life, they have even less incentive to offer low speeds. If gigabit plans are on the table, there's virtually no difference in the maintenance cost for the lower end consumers using 1Mbps, 10Mbps or 100Mbps. It's really only the top few percent of customers who use enough bandwidth to affect QOS. Hence, almost all of these companies offer just a few pricing tiers, the top tier for high bandwidth whales, the middle tier for almost everybody else, and a bottom tier for the super-budget conscious (and also to make the middle tier users feel like they are paying for a premium service.) As the top tier offering gets ratcheted up by technology, so will other tiers. And so will your bill, if you're suffering under a monopoly.