No engine with rich tools ever truly complete. Same goes for 3D apps. If you have such mentality, you are missing out on 3rd party tools that can help you get job done faster, with better results, and sometimes simply get it done (cuz you wouldn’t be able to do that without 3rd party tools).
Thinking along the same lines using UE4 and Unity is already a risk - companies can change licensing, etc. so unless you are using your own engine, you are at someone else mercy.
But that a wrong mentality to have. If you are planning on working on your project for the next 5-10 years, you better rething what you are doing. And I don’t see any AAA companies asking Epic to add C# - they just use C++ and BP.
I don’t need to work at Epic to see they are spread thin - just read forums and see the progress. They have AAA studios to support, own games they are working on and engine development.
And what makes you think they will switch to UE4 if UE4 gets textual scripting language?
Devs who are comfortable with Unity for example and deeply invested into it won’t switch. Why would they if engine works for them already and perhaps they have solid code base to work with?!