Why replacing Churchill with John Buchan is underspecified

What changes if John Buchan replaces Winston Churchill from 1905?

The scenario is too underspecified because Buchan lacked Churchill's political path, and those institutional differences determine what he could change.

John Buchan was a young barrister and Milner associate in 1905, with a political position very different from Winston Churchill's. Moving him into Churchill's career changes party affiliation, campaigning, patronage, and the route to office. The comparison with the 1940 premiership narrows the realistic choice to Churchill or Halifax because both possessed public standing, Commons support, and international credibility. A sound counterfactual needs a plausible institutional path for the substituted figure.