Simple disuse does not mean it decreases the chance of survivability on the part of a species.
Disuse would remove selection on mutations which reduce the functionality of the organ. That is the whole point.
I can't see why still having a cecum would decrease the survivability of the species.
A caecum full of bacteria with little to eat would cause the commensal bacteria to die off. This would open niches for pathogenic organisms such as certain Clostridial species which currently cause serious colitis in humans.
I also find it hard to believe that something used to digest food would suddenly change its function into something that has a positive effect on the immune system, maintains gut flora, and also helps fetal development by producing hormones.
It's minor contribution to the immune system is outweighed by it's propensity to house pathogenic organisms. The reason that the HVA is flooded with white cells is that it is a good place for pathogenic organisms to get a foothold in the gut. It's a bit like claiming that the US border is a great barrier to immigration due to the number of border agents found in the area.
So instead of becoming a vestigal organ which usually connotes a decrease in functionality, the appendix actually took on multiple positive new roles and probably has more functionality than the original cecum
And those roles are . . . ?
There is a question of whether or not it is vestigal . . . .
Not among biologists or anyone familiar with the evidence.
Sure, if you start to use expanded definitions of vestigality and believe that vestigal organs can actually increase in functionality from their previous incarnations which goes against the conventional wisdom
Expanded definitions? Here is the definition laid out by Darwin:
An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose, and remain perfectly efficient for the other.... [A]n organ may become rudimentary for its proper purpose, and be used for a distinct object.--Origin of Species
From the very start of the theory this has been the definition.
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