The reality nearest to our own where the impeachment attempt succeeded is the one where the Senate was D-majority that year. Only a few seats would have to be different for that, whereas the counterfactual of republicans voting against one of their own would require a shift in the very elements of politics. Imagine a world where a D-majority legislature impeached a Republican president. Instead of pointing to an election, Democrats would have to point to a 1000 page report that nobody wants to read. Republicans would be calling it a "political move" and the whole party would be unified against its fairness.