Just look through the previous RNC/DNC chairs. They all tend to serve at most at most 4 years (many serve much less).
en.wikipedia.org
en.wikipedia.org
Reince Preibus is the one exception it seems. But he actually goes against what you are saying because he initially was initially against Trump as the nominee in 2016 and criticized him on multiple occasions.
Republican National Committee Chairman Reince Priebus said Friday that Donald Trump is not the head of the Republican Party.
fortune.com
The Republican National Committee chairman suggested party members ignore Donald J. Trump’s attacks because “we all need to get behind the nominee” in the fall.
www.nytimes.com
Although I don't think it's because Trump is some virtuous guy who lets people who oppose him stay in their positions. It was probably more because he was a political amateur and didn't have much clout in the Republican party at the time. But it doesn't help your point at all that the one guy who stayed for two presidential nominations was a guy who didn't like Trump.
Do you think Obama abandoned Tim Keane when he stepped down as DNC chair, even though he won his first election by a wide margin?