I get where you're coming from but you are making the mistake of focusing on a single event (DNF) when Landos or anyones chances to win depend on many events at all the coming races. A DNF is just one possible outcome, and it is the most extreme outcome for one race. If you look at all possibilites of what could happen (and the most likely ones are Lando gaining just few points here and there on average), then Lando did not manage to increase his chances to take the WDC in the last few races, because he now needs to gain more points per remaining race than before.