I think since the boots are constantly flexing in the same places, the W/B membrane wears out earlier than it would, say, in a jacket. I don't think there's any way to repair it, but you could add some water repellance with Nikwax for Fabric & Leather. Waterproof socks could be used to get some extra life out of them, although they're expensive.

My Keen Targhees lasted quite a few years, but were only used on backpacking trips and very occasionally at home. They eventually started leaking. I had several pairs of Lowa boots (Klondike and Renegade) that came apart before the Gore-Tex ever failed.

If you need them for snow camping, new boots are probably in order.