I don't think - at least in my own experience - that there the demographic 'centre or left of centre; actual or potential 'broadsheet [in print or online] reader' is 'dying out'.
But the Guardian is not just 'left' or 'centre left' is it? It's a particular niche which is largely irrelevant to all but a small subset.
As someone said earlier, it's very, very removed from the grassroots left wing movements, particularly the ones attracting young people.
Their readership is falling away. And it's highly unlikely to be falling away to the Mail, the Sun or the Telegraph.
The most likely explanation is that it's falling away to other news outlets, probably online, which actually manage to connect with a left wing readership and it's concerns, hopes etc.
Which probably don't involve the Astor Grill at Cliveden, Seaweed, Gin holidays in Northern Ireland, the best places to eat and drink in Normandy, homes with Tennis Courts or mansions in W Sussex (all recent stories) or people with designer tea sets especially to feed their chihuahua.
I mean, it amazes me that they've managed to get away with this shit for so long whilst claiming to be the voice of the left wing.
Marx and Engels would have told them to shove it up their arses in a nano-second.