JWST development was started in 1996 (25 years ago), so it can't have been in a warehouse for 30 years. The detailed design phase was ongoing in 2007 and assembly of the primary mirror was only completed in 2016 (dates from https://en.wikipedia.org/wiki/James_Webb_Space_Telescope).
Perhaps you're thinking of the National Reconnaissance Office telescopes that were offered to NASA in 2012 (https://en.wikipedia.org/wiki/2012_National_Reconnaissance_O...)? Those are similar telescopes to Hubble in some ways, and one is being used for the WFIRST mission.
Despite sometimes being billed as such, JWST can only be partly considered a replacement for HST. HST has ultraviolet observing capabilities (wavelengths a short as ~90nm), while JWST observes primarily in the infrared, with some capability to wavelengths of 600nm. There's some overlap (in the 600nm-1600nm range), but JWST can observe out to about 28,000nm (28micron) wavelengths. The JWST wavelength range is perhaps more comparable to that of the Spitzer Space Telescope (particularly the IRAC and IRS instruments, covering 3.5-40microns; MIPS covered out to 160microns). The LUVOIR concept (https://en.wikipedia.org/wiki/Large_Ultraviolet_Optical_Infr...) would probably have the better claim to being an HST successor, given its proposed UV capabilities.