They can, and they have. Some of the religions that claimed to be word of God have already been abandoned. I do not think that this is what worship of Saturn was about. Saturn was more of a god of vegetation. Saturnalia haven't been fully abandoned, but they have resemblances in modern Carnival.
I don't think that religion is really meant to possess all the answer. Science wants to get there step by step. Religion is not about that.
I do not really think that shamanism or sympathetic magic were meant to be interpreted as word of God. Some religions personify God, like Christianity you mentioned. Other do not really do that. I myself think that personification of God in the form of a king, and killing them at the end of some fixed term, or at first signs of their old age, is not a good idea. I think it hasn't been always this way, and killing God is being gradually abandoned. Muslims you mentioned do not kill or even personify God at all, but they deify Muhammad, which is hardly distinguishable from it. Sikhs identify God with a book, and perhaps with the ten gurus to some extent. Modern religions such as Wicca still practice mock sacrifice of God, but other modern religions do abandon the concept. At Rainbow Gathering no human sacrifice, real or sham, is being practiced.
I am surprised that you do not understand that old religions are being abandoned when they no longer serve their purpose. Do you think that once a religion is constructed a people is compelled to practice it forever?
Oh yes, I understand that. I thought you were trying to defend modern religions. It is difficult to see that as happening today (though yes hopefully in the future) when people are beheaded, burned at the stake, etc for challenging or asking questions about some belief. This is very common with Islam today. It has had periods of such actions within "Christianity" as well. That dreaded group think combined with human nature often leads to us vs them situations and bigotry of one form or another.
Now another thing. I think you might be equating some things that are considered spiritualism with religion. Spiritualism is not really organized. Thus it isn't technically a religion. It tends to be personal for each person rather than pushing their beliefs and converting other people.
Some of the things you referred to here actually fit within spiritualism rather than religion.
Spiritualism is still not scientific, but as I stated there are many things science cannot explain. If it can't repeat it, observe it, etc then science can't do anything with it. That doesn't mean it does not exist.