Do you usually plan your sailing routes over 10 days ? Beyond a 5-day forecast I prefer in any case to rely on a more recent forecast. But of course it is more pleasant to have a 10-day forecast available.
If you do not take the Premium subscription, you still have access to the different layers beyond 6 days even if they are overwritten by the mention “Windy Premium”, in very clear tone. So it’s up to you…