There are always a few who only start to follow during events like this and whilst many will not stick around when the shit times come back, some will remain.
I grew up in a mostly non sport household - in that my parents and a lot of my family have very little interest in sport at all. Big events like this reach out to some people who otherwise have very little access to the sport.
Prior to 2016 there were probably people in Wales who had never considered supporting the football team - or felt their welshness was defined by their support for the rugby side. If you previously saw the Wales team as a pale imitation of the England side then I can understand how you might not be attracted to that. However now we have had more exposure I think people can see that supporting Wales has a very different vibe to supporting England.