Build vs. buy is pretty much never a question of developer quality, but a question of ROI. Even with the best devs in the world, building something on par with Unreal (or other commercial engines) is a huge outlay of capital. With relatively little return over a single game, you need multiple hits to pay off the engine.
There aren't a lot of companies that can afford that.
Here's a decent write-up of some of the major issues that actually plagued that project, as opposed to "bad devs": https://blog.devgenius.io/cyberpunk-2077-through-the-lens-of...
All of those issues were there for their last project as well, but somehow they only failed this time even though they had a much larger budget and larger team with more developers. The most reasonable explanation is "bad devs", even though that will never be an official reason. When you swell the team size quality tend to take a hit, and when the old good devs becomes managers and the new devs try to replicate the work you often get problems.
That article describes how you need to work if you have bad developers. CDP-R assumed that they still had as good developers as they had previously, which wasn't true and that bit them.
All the indicators point to "scale". Having worked in games for almost two decades and having seen anything from solo dev to teams of hundreds, and the growth in between, my experience leads me to say "scale" as well.
Do you have any indicators for the "bad devs" problem? So far, you're just stating it as if it were categorical truth.
https://www.cbc.ca/news/business/sony-cyberpunk-2077-playsta...
Edit with more arguments:
You could of course blame managers for that, but delaying a game since your bad developers aren't capable of delivering isn't really an option either. How many years will it take those developers to get things right? What they would need is to delay the game and replace the developers with good developers, or scale back the complexity of the game to a level those developers can handle.
The company in question choose the latter, they said they will use unreal engine for future games instead of an inhouse engine. So they admitted their developers aren't good enough.