I can advertise for a "React developer" and find people familiar with react rendering. I will never be able to do that with in-house render solution. This will apply to whatever replaces react as well.

Unless I'm selling a render solution, I don't want to be maintaining a render solution. Maintaining glue code might suck, but writing everything from scratch limits the scale of the work that a person can accomplish. I liken it to polishing gears rather than driving the car.