"The ISPs customers are paying for a connection to the ISPs network, nothing more."
They are also (usually) paying for a specific connection speed. If a customer is paying for a 100mbps connection and Hertzner has 100mbps of spare bandwidth, but the ISP consistently only has capacity to relay 20mbps per second from Hertzner to my computer, then I would argue that it is the ISP's responsibility to upgrade their hardware so that they can fulfil their obligations to customers that they have sold 100mbps connections to.
They are also (usually) paying for a specific connection speed. If a customer is paying for a 100mbps connection and Hertzner has 100mbps of spare bandwidth, but the ISP consistently only has capacity to relay 20mbps per second from Hertzner to my computer, then I would argue that it is the ISP's responsibility to upgrade their hardware so that they can fulfil their obligations to customers that they have sold 100mbps connections to.