I remember reading articles about the Hubble replacement being built.
I’m going to search the SDMB archive also to see how long before 2021 it was mentioned.
From 2007: Technical Article on the James Webb Telescope
2014: How many planets can the James Webb telescope analyze for life in one day?
2005: NASA can really suck sometimes - #18 by RandomLetters
2005: White House to Hubble..So long..it's been nice. - #17 by Dead_Badger
2020: NASA Space telescope 10 years late and $9 billion over budget (and counting) & Cost overruns.... - #3 by Hermitian
This is not all of them, just the first 6 I found.