Your solution, if I understood it correctly, violates the main idea of timelines, as it creates an "alternative" version for timeline N. It's possible that primary finished contrecord and whole segment was archived. Currently, replica always has subset of timeline N, so it's safe to start new timeline from any point. But with your timeline finalization logic it will create its own version timeline N that will conflict with already archived version. If replica will archive timeline N+1 you may end up with two different "tails" of timeline N in the same archive. Also I assume some third party tools that rely on WAL may be broken by such change.
