LaFollette ran with the support of the Socialists in 1924, after Wilson's crackdown and Debs had gone to jail. Flash back to 1912, and without Debs the Socialists would have less support than 6%. I don't think they would support TR (Roosevelt considered New Nationalism the "antidote" to socialism), but they may give some support to Progressives in House races or more local races. So Progressives could do better, but still lose that year and again in 1924.
True, but that's all assuming that Debs was essential to Socialist growth in the United States, and I'm not entirely sure that that is the case. He was certainly important and he did a lot to try to Americanize Socialism to make it palpable to the American electoral system, but he wasn't the only one doing this (and, as previously mentioned, Victor Berger was possibly far more influential - both on the local level in Milwaukee, but also throughout the nation. And, for all of his strength, Debs actually made some moves that harmed the party, such as his support for the creation of the IWW. Its entirely possible, that an alternative leader would have actually strengthened the party.
As for LaFollette, yes, he did have Socialist support in '24 (and for that matter, in his runs for the Senate as well). I'm not sure if Debs' absence would help or hinder his run in '24 - though I actually suspect that butterflies of a lack of Debs would lead to a very different playing field by the time that election rolls around. Hell, Debs moving back to the Democrats after the collapse of Populism might lead to him getting the Dems nomination in 08, 12, or any other year. I believe
@David T wrote about just such a possibility back in the SHWI days. Not to hijack the thread, but LaFollette's best chance to the presidency was in 08 or 12 in any case