A tale of two missions...
Once upon a time, times were good and a space agency planned an ambitious mission.
A very ambitious mission. It would use never-before-built optics made with unequalled precision, and sensors which only existed as science fictional concepts in the heads of ambitious young engineers.
After several years, NASA decided the mission was too big and split it into two missions, one primarily for imaging, the other optimised for spectroscopy.
Then times got hard, and the two missions were merged, the mirror was descoped, the instruments were cut and compromised, the spectroscopy was compromised in exchange for imaging optimisation.
Finally, more than two decades after it was proposed, after 15 years of development, it was launched. And it did very well indeed. Still does. It may keep working and doing breakthrough (and bread'n'butter) science for more than a decade more.
And, shortly after it launched another agency launched a spectroscopy optimised mission which compliments the imaging mission relatively well.
The original spectroscopic instrument was moved to a mission for yet another agency, which failed... so they built it again and launched it again... where it failed again (just the instrument this time...). That was 30 years of trying, and in the meantime most of what was to be done got done in pieces and with other missions.
Who knows, maybe history will repeat itself.
It is just that 20 years is an awful long time to wait.