The words of Christ are always for universal application. At the same time, within those letters, there are specific issues which are within the lives of individual Christians and individual churches. But they are not limited to any specific time. All Christians and all churches for the last 2,000+ years have needed those messages. And yes, believers are to be overcomers by the power of the Holy Spirit.But why put them with the Apocalypse and say we are to overcome... unless they are specifically targeted to a specific people at a specific time.
False doctrines and false practices had already crept into the churches while the apostles were on earth. So those seven churches are a microcosm of all churches in all ages. The real issue is whether those warnings are being taken to heart by the churches which surround us.