UKIP was steadily increasing its share of the vote with its single issue stance. It was top performer in the Euro elections and almost got a lot of MPs in 2015 (at the expense of the Tories). Cameron was dead scared of them and thought that a referendum defeat would cause them to lose...
In about 100 years time, when someone writes a history of the UK in the 21st century (assuming there is still someone around to write it - and can write) I would guess that Farage and UKIP would have a chapter of their own, in much the same way that the Chartists or the Corn Law protesters had...