To me this would be an argument against a tightly coupled monolithic engine like UE4. For instance, imagine if Valorant is only interested in a new version of the engine for new post-processing effects introduced into the renderer. Why should they have to migrate gameplay code to get benefits which have nothing to do with gameplay?
They actually _are_ changing things like the renderer, hence their need to take a long time for migrations. Basic gameplay code is written with blueprints, base classes are in C++ but do not replace the core code.
