Like almost all the guff about Brexit, this doesn't mean anything in fact.
Yes demographics are changing - and the earth is still orbiting the sun, but it may not always do so.
The simple fact is, society evolves as people die, and new generations with new thoughts arrive, and their views are the norm for them.
Do we think twice about the fact that slavery is abolished, we don't hang children for stealing bread, we don't have children dying in mill machinery or climbing up chimneys? No - because these are norms to us.
In the future, a society that may think re-joining the EU is a good idea may be the norm, or we might all be living on Mars so it won't matter anyway.
It's all speculation, it's free, anyone can do it, and you don't need real evidence because it's made up, and no-one knows what's going to happen, so media types get paid to guess and ponder - to no end whatsoever because we don't know the future.
Is it a gloomy predication - no, it's another meaningless puff piece that gets a journalist's face on social media - something else our ancestors didn't have to think about!