I think you might be underestimating the complexity of the JWST. A publicly funded project of this complexity will almost always have budget overruns and delays.
So much of the complexity and cost overrun of the Webb was because we lacked a heavy rocket with a large payload fairing. The need to fold the telescope so delicately and intricately was the biggest challenge. Hopefully the larger rockets in development mean that future telescopes will not suffer from such constraints.