When you write:
It seems like most adaptations for bipedalism would work well enough in other apes, and there should be at least some pressure to move well on the ground
Based on our understanding of how bipedalism arose in ape-like human ancestors, I don't believe that this is true.
- There are significant constraints that can make bipedalism disadvantageous (at least in the case of humans, see below), and thus apes would have to either find a substantially different bipedal strategy (likely difficult, given the path taken by human ancestors), or accept the tradeoffs of the human approach to bipedalism.
- Do we know that moving around on the ground on all fours is a poor strategy for these primates, to the extent that they need to do so?
Constraints of human approach to bipedalism
Pelvic adaptations to bipedalism significantly restrict the size of the birth canal (see image), among other changes. This imposes a significant tradeoff between walking upright, maternal/neonatal mortality, and other parameters.
Quote from the linked article:
Pelvic anatomy impacts human performance. To walk upright in an energetically efficient manner with a minimal risk of injury, the pelvis must be robust and have a shape that maximizes muscle lever arms and minimizes load [5–11]. The ability to regulate body temperature is affected by the width and depth of the pelvis, which plays a crucial role in determining overall body proportions and the body's surface area-to-mass ratio, thereby influencing heat loss through the body's surface [12,13]. Finally, and most critically from a selective standpoint, pelvic shape must allow the delivery of a healthy infant without harm to the mother . These goals are not all achieved by the same pelvic morphology, yet all three demands must be met. Therefore, selection has favoured compromises between these often contradictory pressures [15–17].