I dont think its cynical at all, as someone who has done some work for political campaigns, follows them closely, and studies them. Incumbents keep power within their own party by generally massively outspending their primary opponents, making it extremely rare for them to lose in the primary. The only real exceptions to this recently have been Dem stronghold incumbents losing to progressives.
After that, its just whether or not the incumbent can defend against the opposition party in the general election. Primaries are generally where the biggest changes are made, and are also where its the hardest to oust the incumbent.
