I am 100% sure that you can design it not to leak. There have been bigger engineering challenges in the history of man. It is just the fact that there is no simple and economical solution that causes manufacturers to keep making leaky boats. For instance, at great expense, the spar could be custom manufactured with a welded in aluminum dam inside at the lowest halyard exit, or an epoxy dam or the like. For outside leaks, the spar could have a welded on skirt with a matching labyrinth seal profile integral to the deck.
It doesn't have to leak, that's the point.
The question is how bad does the leak bother you? $5000 bad?
Perhaps a more well designed mast boot is in order. If it was sealed at the top with a hose clamp and there was a steel ring bolted to the deck, a UV resistant boot would be easy to design. It's just the extra bother and expense that stops these designs from being realized.
Along with the fact that my mast is deck stepped.