I would hope game engine systems would follow Open-Close principle and not require modifying the actual engine much (eg. not forward compatible with updates without large merges).
The open-closed principle does not matter much when the release is in 1 month, your game is running at 60% of the target FPS and you know that it won't be updated after release because it already got bad reviews
Bigger teams using an off the shelf engine will likely end up modifying large portions of the underlying engine code-base in order to implement what makes their games different from other games in the market.