A lot of complexity of JWST is driven by the very restrictive mass and size limits. If you get a wider rocket and a 50% larger mass budget, then you can skip a lot of the complex unfolding and you can probably make a comparable diameter space telescope for literally 10 times less effort, cost and time.
The available launch vehicles probably demanded a certain amount of ambition in the plans. You can't launch small experimental telescopes with a short planned operation on the shuttle. It's too expensive just to show up with anything less than Hubble's successor. Now the new space race is aiming at making access to space cheap and common.
Whatever follows probably won't be a big telescope. It'll be all the little experiments that couldn't find funding over those 30 years because there was nothing to launch them without breaking the budget. There's a lot of progress deferred because every launch had to be big.
It will be another infrared telescope. Hubble's capabilities have been all but surpassed by ground-based optics. Some of the new giant telescopes already being built will easily surpass Hubble (100+foot mirrors etc). What JWST does is see in infrared, something that ground-based telescopes can never do as IR is absorbed by atmosphere. So whatever replaces JWST will likely be another, bigger, IR telescope.
>>With its 30m diameter prime-mirror, TMT will be three times as wide, with nine times more area, than the largest currently existing visible-light telescope in the world. This will provide an unparalleled resolution, with TMT images more than 12 times sharper than those from the Hubble Space Telescope.
Granted a lot of that was scope creep but nonetheless, I'm betting it's not launch capacity that's the bottleneck.