For a start Labour are not dead. They are struggling. They were struggling before Corbyn became leader. At least in 2017 Corbyn’s Labour forced a hung Parliament.

The problem for Labour is not down to any one individual. It is a tale of two referenda.
In Scotland the 2014 Indy referendum resulted in a surge of Scottish Nationalism and it came at Labour’s expense with the SNP supplanting them as the main party whilst the Tories kept hold of the Unionist vote.

Then the 2016 EU Referendum left them in a very difficult position as a Remain party as much of their traditional support in the north wanted out of the EU
Brexit has then resulted in a surge of English nationalism which Johnson has taken advantage of. The Tories had no such problem because they were primarily a party that ended up supporting Leave which was supported by the vast majority of Tory voters.

So even though Cameron and the Tories handled both referenda very badly they got away with it. In that sense Labour have been a bit unlucky