"Aligning the primary mirror segments as though they are a single large mirror means each mirror is aligned to 1/10,000th the thickness of a human hair. This alignment has to be done at 50 degrees above absolute zero! [0]
[0]https://www.nasa.gov/topics/technology/features/webb-actuato...
1. Where doe the telescope deploy? Ideally in LEO so you have a chance fo fix things but then that complicated the launch. You have to park in orbit and restart th eengine at a later point. AFAIK JWST didn't enter a parking orbit so would've required additional fuel. That might not have been possible;
2. What if the launcher itself fails? Do you want to have the capacity to launch another rocket, have it rock with JWST and then go on? If so, that's a whole new level of complexity and a set of problesm you have to solve as well as things that can go wrong;
3. If you look at the flight plan there are a bunch of turns. How would the G-forces affect, say, the deployed heat shield?
4. JWST actually had to rotate during launch to point the instruments away from the Sun so as not to destroy them. This already added complexity. Doing this with the deployed spacecraft would probably only further complicate this.
Is it impossible to build to a lower level of reliability then have humans available to fix/adjust things in orbit?
Then once it's fully assembled in place, move it to its final location?