Why will a Tzadik never falter?
Rashi: When he falters, it is not a permanent faltering, for he falls and rises.
R. Yonah: Hashem is a fortress for him in all his afflictions. This adds explanation to "Ma'oz la'Tom Derech Hashem" (29).
Malbim: Faltering is due to a reason - weakness of legs or the place [where he stands] is inclined. Even if a Tzadik falters because he did not stand on a good place, his faltering is not permanent, "Ki Sheva Yipol Tzadik v'Kam" (24:16).
Why will Resha'im not dwell in the land?
R. Yonah: Their good will not last. The purpose of their serenity is to destroy them. Since it is clear that this is their end, just it is not known when, "Derech Hashem Mechitah l'Fo'alei Aven" (29).
Malbim: Even when they dwell on [flat] land, not on an incline, rather, a fixed dwelling, they will not remain dwelling there. Resha'im will be shaken from it.
