Knobs get conquered in a number of ways. Beyond the obvious — actually being de-knobbed — two things happen to render them obsolete.
One, is that skateboarding evolves. The cliché goes something like: “without struggle, there is no progress.” When spots are knobbed, our most able-bodied athletes see an invitation to have quicker feet and longer ollies. This can be seen via people skating past the knobs at Pyramid Ledges, and between the knobs at Verizon Banks — though it will take a generation of people thinking Tiago is “normal” before the re-knobbed Veterans manny pad gets unlocked again. (Its first post-knob unlocking occured under the supervision of Anthony Pappalardo in the 1870s, A.K.A. in Fully Flared.)
Option two is playing the long game. You wait for weather, rust and general wear-n’-tear to do their things.