To be honest, the first two lines don't make sense when used together. It sounds like human beings were called "computers" but then it says that electonric devices did the work of people. This juxtaposition makes what comes next -- again referring to humans -- confusing.
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Grammar GeekWhen you say "It's believed" you mean that you don't have any facts to support this. It doesn't seem to apply in this case.Thanks a lot, GG!
The addition of "but" is okay, but it makes your writing more informal, because truly formal writing does not use conjunctions to begin a sentence.