If TR died, the Progressives would be more likely to support someone associated with him, like Hiram Johnson, rather than La Follette. And in any event, if the Socialists weren't satisfied with Bryan in 1908, there is no reason to think they will find La Follette sufficiently radical for them in 1912. Even the so-called right wing of the SP was heavily influenced by the orthodox Marxist argument that La Follette-style trust busting was futile, that trusts were a natural development of capitalism, and that the answer to the trust problem was neither TR-style legalization and regulation nor La Follette-Brandeis style trust-busting, but for the government to own the trusts. (The difference between so-called right-wingers like Berger and the left was that Berger favored the government *buying* the trusts instead of seizing them without compensation.) And then too the immense popularity of Debs among Socialists of all stripes would make it impossible for them to give up on his candidacy.
The reason Socialists weren't satisfied with Bryan not because he wasn't radical enough because many of them still remembered the collapse of the Populist Party because of Bryan fusion with the Democrats in 1896. Also Deb's wasn't the most enthusiastic about his own continued candidacy for the Socialist Party, for example in 1908 he attempted to get Bill Haywood to accept the nomination for the party and in 1916 decided to pursue a congressional seat instead of the presidency. It was reported that after every presidential campaign Deb's would spend weeks recovering from said campaigns because of exhaustion from the rigors of the campaign. It is not necessarily out the realm of possibility for Debs to abdicate from seeking the presidency. Also Debs after he was released from prison was a prominent backer of the CPPA and LaFollette's presidential campaign in 1924.