The Seven Churches are the historical context of Revelation.

If you are going to downgrade the scale of what the book is describing to being not the entire world or even the entire Roman Empire but primarily one Province of that Empire, it has to be Asia.

But 70 AD Preterists are actually a lot like Pre-Trib Dispensationalists in that they can't accept Revelation as being geographically about anywhere other then exactly where the Old Testament is about.  And so in a Preterist Facebook Group I saw someone talking about how "Symbolic" Revelation is and that includes the identities of the Seven Churches.  That is completely backwards.

The Structure of Revelation is that the Symbolic Vision starts in Revelation 4:1 and ends about 22:5.  The more direct messages to the Seven Churches do use symbolic language sometimes but that's different, a lot of it is making references to Old Testament stories as analogues to current scenarios the same way one might reference a movie today.  Those references in chapter 2 and 3 do help contextualize how similar references will be used later.  

It is the things that seem like geographical references to the Old Testament Holy Land that are meant to be filtered thorough New Testament Doctrines. 

Every reference to The Temple or Tabernacle is meant to be understood in the context of Stephen, Paul and Peter's teachings about all Believers being The Temple of God.

The name of Jerusalem is used in Revelation only in reference to New Jerusalem which is clearly Paul's "Jerusalem of Above which is the mother of us all" from Galatians 4:26 and Hebrews 12:22, likewise the Sion of Revelation 14 is the Heavenly Sion of that same passage.  (As someone who holds the view that the NT authors were also a little Stoic I suspect Stoic Cosmopolitanism is also an influence on how Paul and Revelation talk about Jerusalem and Zion and the Beloved City of Revelation 20:9 and Romans 9:25.  But that's secondary to the Scriptural Points.)

Babylon in the Hebrew Bible is the mother city of the great Pagan Gentile Empires, so it's symbolic usage is likewise of the New Testament era's great Pagan Gentile Empire.

I don't claim to know exactly how to decode everything yet, but all of that needs to be the starting premise. 

There is no logic to using cities in Asia as symbols for cities of Judea.  If some stuff can't be interpreted in any other way then as references to a region outside of the Roman Province of Asia, then it proves that the scope of what the Book is foretelling isn't merely local.