Actually, the problem with my statement is not that it is implausible, but that it is practically tautological. Animals with better abilities to move around on land have better abilities to move around on land. The right time to develop something better is anytime, so timing is not difficult. Humans developed stronger hip joints for better bipedal walking and running almost 250 million years later than the first archosaurs did. And there are many different ways to improve limb function, in part because “improve” is extremely vague - a limb that is not as good for one thing can be better for another. Evolution is about “whatever works”; the “survival of the fittest” is a rather inaccurate phrase in most cases.