The seven churches = seven eras of history interpretation isn’t actually an evangelical thing. It goes back to medieval times. But it’s never been particularly satisfactory as an interpretation, because the problems and strengths of the seven churches are so universal. As a lot of people have pointed out, it makes more sense to make a connection to St. Paul’s letters to various churches, or to remember that seven is the number of completion and thus 7 churches symbolize the One Church as a whole.
Now, the medieval interpretation probably comes from St. Augustine’s seven eras of history, which show up in many of his books and treatises. But he counted the eras of history as starting from Adam, with only a few Christian times included in the eras. Obviously that didn’t fit the seven churches very well, so people made up their own. And as time has advanced, people continue to change the length of their eras.
The seven churches were historical churches with historical problems. The best way for most Christians to interpret the seven letters to the seven churches is the obvious way that most prophecy works – as moral instruction and dire warning. Do the good things that the members of the seven churches were praised for doing. Avoid doing the bad things, or stop doing them and repent.
People talk about Revelation being confusing, but then they ignore the parts that are simple.
