Why aren't Christians Jews? Why aren't Jews all pagans worshipping the Canaanite pantheon? Why are there 41,000 different versions of Christianity according to
Pew Forum?
In relation to your ideas, it would seem to me that all these questions can be seen two ways, depending on how you look at them.
1) They can be viewed as data points supporting your assertion that "religion . . . does not allow for changes," and therefore people need to create new versions of their religion to incorporate changes.
or
2) we see that religions do in fact change and evolve over time by the fact that a few religions have spawned entirely new religions altogether and many different variants of the same religion, challenging the idea that religion does not allow for changes.