Part of the charm of AH is the same as regular historical fiction, ie the ability to use historical personages or events in your story. The closer you stay to real history the more you can do this, so it will tend to be conservative as to what develops in the OTL vs the ATL. Were you to follow what should logically happen in most cases after the POD you would be writing pure fantasy in less than 50 years.
'Course that can be part of its charm too, that way requires less meticulous research and more "informed imagination". The first type is more prevalent because it requires more hard work and writers thus think its more "honest", since most don't have any confidence in themselves and their imaginations
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