Imagine a 5 test series against Bangladesh. It would be a disaster. Just read an article saying that the ICC is thinking of a Test Final between the top 2 ranked Test Nations every few years. That should be a 5 test series for sure!
I guess they are considering Martin Crowe's test championship formula seriously with a test semifinal and final. It would be great and it would make people sit up and watch test cricket that does not feature their home nation too (if the result impacts their home nation's progress).
But a 5-test series final? 3-test final would be enough.