Wouldn’t incumbents losing suggest that gerrymandering makes elections less competitive and favors incumbents?

Well, after redistricting by supposedly partisan committees one would expect the redistricting process to further entrench incumbents because either party would be unwilling to yield established seats. In fact, the opposite happens.

Wouldn’t incumbents losing after partisan gerrymandering suggest that current gerrymandering favors parties so much that they can’t improve the system in ways that benefit them specifically, and that even a partisan committee led redistricting would be more representative of the voters more will than the current districts?