The 100% c-bet strategy was at the outset an (extremely) exploitative strategy based on that people fold to much. It doesn't always work well any more, even at 5c/Tc, there are a bunch of players check-raising the hell out of you on boards they see that you have missed. Whether I c-bet missed nasty boards nowadays depends on the opponent. I don't know if these same players have taken it to the point where they check-back themselves on these types of boards, hence necessitating donking. But maybe donking is really needed already at 2/4 6-max.
If this isn't all my imagination, then this is kind of scary. There are a few hundreds, max 1000 people regularly playing, and even the fish are getting harder to beat. Fish, by the way, the ones that actually win a little, thrive on too high c-betting from TAG/LAG aggressors (in addition to beating their fellow fishae).
Some more of Yohan's whimsical theories: Forgetting ranges and all for a while, there are many players that handle donk bets incorrectly. They raise donk bets (regardless of holding) because they hate donks. Against such players, a donk with strong hands may pay better than a check-raise on the flop (you call the raise on the flop and check-raise the turn, they still might not believe you). This is most useful in small stakes very deep stack NL (versus the right players holding an overpair of course ♦), but works to some extent in FL as well.
Drawing cheaply: Versus a PS standard player that mostly just call a flop donk, one could consider donking good draws that aren't quite in the check-raise bluff range, for instance, an king high or ace high flush draw, like A♠ 8♠, on a small board as 2♠ 7♠ 9♥. A blank turn is then checked. It's very likely that it gets checked back, since the opponent now might put you on a small pair and might not bet two high cards (better than ours). If this succeeds, drawing two cards to a flush costs 1 SB. [True enough, a check-raise may still have better EV seing this holding in isolation. The problem is that one cannot check-raise every draw because that leads to a highly unbalanced range. See this as an alternative to check-calling.]