We'll see. I would definitely contend that they would not have done what they've just done without Farage. And as far as the vast majority of people are concerned, UKIP is Farage. I doubt many people outside of hard-core supporters or political saddoes like us on here could name a single other. Perhaps 'that Tory changeling with the weird eyes'.
Farage was extremely effective and energetic. Perhaps they will find someone else who can be effective, but I don't think it's a given, and if there is some kind of in/out referendum on the EU (which I suspect would vote 'in' by a bit of a margin) then UKIP may be seen as an irrelevance at the next election, campaigning on the basis of something that everyone knows won't happen.