Well the Whigs didn't die off; they merged with the Radicals and the Peelites to form the Liberal Party, who later merged with the SDP to form the Liberal Democrats.

I think a more realistic objective is for the combination of the Tories' broader decline plus the Tory/Reform split to knock them out of being a top two party in England. FPTP is unrelenting - I can just about imagine a scenario like the Liberals experienced in the early 20th century, where the Lloyd George/Asquith split and the trauma of governing through the First World War took them from the largest party prior to the 1918 election, to the third party following the 1922 election.

In that ideal scenario, we'd see a two-pronged squeeze where the Lib Dems supplant the Tories as the party of the middle class South of England and Outer London, and Labour supplant them in their North and Midlands seats. Once the Tories are no longer part of the FPTP duopoly, the electoral system makes it very difficult for a third party to ever get back to where it was.