I don't think that holds true at all. If you look at pg_stat_bgwriter in any remotely bugs cluster with a hot data set over shared_buffers you'll notice that a large percentage of writes will have been done by backends themselves.
Even if what you are saying it true, which I am sure is, the pages that the backend is evicting mustn't be recently used by the LRU algorithm which means that the WAL pertaining to the last change to the page in most likelihood is already replicated, unless the replication is really lagging much behind. Of course, if the standby is not able to keep pace with the master in a realistic manner then we have a problem with the approach.