I am comparing two textbook's presentations of capital structure and Modigliani & Miller propositions. The first one is Berk & DeMarzo "Corporate Finance" (5th global ed., 2019), the second one is Hillier et al. "Fundamentals of Corporate Finance" (3rd ed., 2017) (here is a link to a slightly different edition).
They seem to be illustrating the same thing but quite differently. So I guess it is not the same thing after all. What is the difference between the two books' setups/assumptions that makes the graphs look so different?
(Also, does it make sense that in the bottom graph (Hillier et al.), $R_D$ stays constant irrespective of the level of $D/E$? Should the lenders not require higher compensation if $D/E$ is higher?)