Not ALWAYS, see the LCMS and PCA and WELS that are considered ‘mainline’ for some reason.
The LCMS was founded in 1847, WELS in 1850. They are ‘mainline’ in the sense that they are the stubborn old original synods that stood against the popular WCC and NCC nonsense; when the libs couldn’t have their way, the libs stormed out to form their own wishy washy churches, often larger than the old originals.