I didnt say that it would suceed. But by 1914 the sozialist were becoming the leading power of the Reichtag. I dont assume that this election result would change for the worse for the socialists. The conservatives might prefer either fascism - or some other rightist-absolutist idea to socialists. Part of the liberals - especially the monetarily better off when facing rule by socialism might prefer - i dont say they would - a rightist absolutist regime that leaves them more money.
So when faced by a socialist led state - which they were - the rightist antidemocrats especially if they get a charismatic leader might have enough of a base to give it at least a try.
And mostly Im not sure that the yunkers will go down without a fight.
I agree that Germany is the least likely to go down this path but i wouldnt say its impossible.
The SPD, through gerrymandering and a restricted franchise "only" became the strongest party by a relatively small margin despite a winning the popular vote by a much bigger margin. So the SPD has to reach electoral and constitutional reform if they ever hope to reach the chancellorship.
This can only happen in cooperation with both left- and right wing liberals. So the SPD has no real chance of realizing their economic goals for at least a decade or two.
In a surviving German Empire, the historical shift of the SPD away from marxism to left-center social democracy is likely to happen much faster than in Weimar and the FRG. This is caused by demographic, economic and systemic trends.
So if the SPD is in a position to seriously "take over" they are probably relatively tame center-left. The SPD/USPD split is still going to happen, Luxemburg won't stay in a reformist SPD forever.
Demographic and economic trends will force the SPD to broaden their electoral appeal. White collar workers will rapidly grow as a demographic, the same is true for high qualified industry jobs. The SPD as a marxist special interest party for the "industrial proletariat" won't be able to win elections in the 30s, especially not if we see another more radical party on the left.
And a left-center progressive party there is no way that the right would try to overthrow the democratic order. Not after decades with a working parliament.