Putting a device into space is relatively cheap for a project like this, and is only dropping with each year. Say $100M - $200M, on a $10B project, puts the launch at just 1-2% of the price tag. I believe the Hubble total project cost is pushing past $16B in adjusted dollars.
It's the massive capital expenditure on the telescope and its infrastructure that is the significant limiting factor on the pace of putting newer and better telescopes in place.
Apparently for the cost of all the Shuttle trips to Hubble, they could have just put up another Hubble without its problems.
Same thing in this case: instead of visiting Webb, they could build a new Webb without all the origami, and assemble it by hand in orbit for a tiny fraction of the cost, and end up with two of them. Then, maybe put different instruments on the second one.
It's the massive capital expenditure on the telescope and its infrastructure that is the significant limiting factor on the pace of putting newer and better telescopes in place.