It doesn't need to sell out though, it's still making shedloads of money. Whether something is a sell-out or not is no indication whatsoever of its viability. Plus it doesn't always sell out on Broadway either.
Wicked now sells on word of mouth and the name alone, and it's not the first time it's extended for this length of time. So I personally don't think the tour will affect sales that much, it's still filling a decent proportion of one of the largest theatres in the West End 8 times a week. It hasn't sold out regularly in years - I think I've only attended a sell-out performance once and I first saw it in 2007 - but it's still there, and it will be for a few years yet. I imagine they won't even start properly considering closure until it's reached 10 years. And then we might just have a WWRY situation of announcing closure and changing their minds if demand surges. The tour will probably be over by then as well.
Hardly anything truly sells out in London at the minute - Les Mis sometimes off the back of the movie and Book of Mormon off the hype of it opening. Even the Lion King had some empty seats when I saw it recently. It gives completely the wrong impression to focus on whether something sells out or not, even without considering the difference in theatre sizes.