Reading Wikipedia's article on the James Webb telescope, I'm struck by the following information. I don't understand how it arose as an optimal and/or preferred mission choice.
So,JW is in an unstable orbit at Lagrange L2. Unlike many other missions, which can last decades longer than planned because their main need is low level electrical power, JW also needs thruster fuel to regularly ensure it stays in its designated position.
The telescope's nominal mission time is five years, with a goal of ten years .... after a 6-month commissioning phase. JWST needs to use propellant to maintain its halo orbit around L2, which provides an upper limit to its designed lifetime, and it is being designed to carry enough for ten years.
JW launch mass is about 6500 kg. The total mission cost is about $9.7 bn, with thruster fuel for 10 years. The telescope's thruster fuel for orbit maintenance is 238.5L/274kg (159L/159kg hydrazine and 79.5L/115kg dinitrogen tetroxide) at launch.
Other past missions:
Other flagship missions have spectacularly outlived their planned missions.
- Hubble (planned 15, actual 30+)
- Voyager (planned 5, actual 50+)
- Cassini/Huygens (planned 4, actual 20)
Even allowing that some of these past "planned" missions may have been durations for primary mission only, with an extension expected, the overall impression is that missions can outlast and outperform their original planned duration, and be repeatedly extended thereafter, because of their value once deployed.
But JW was deployed with a single and absolute limitation point - its thruster fuel supply, which means that however successful, its duration is limited to apparently 10 years. Not long at all, for NASA's current flagship project, and one that judging by Hubble could still be valuable and quite possibly still working fine, in 20 or 30 years.
Assuming that halo orbit corrections occur at a relatively constant rate, it would have only taken another 274kg to build in capacity for an entire second decade. Around 4.2% total payload increase, for a doubling of viable mission time - and many missions have been valuable after 10 years, one may assume JW would also be. Also new or successor $10bn projects are not readily flicked into space on whim, or on short timescales, and are subject to political uncertainty, so there's a time, opportunity, and cost incentive, to make as much as possible from this one.
I don't imagine the current launch strains Ariane 5 so much that an extra 100 - 300kg was completely non viable.
Even if it did, the payoff of any extra months or years must surely be huge and I'd expect it to have been preferable to find a telescope mass, or launch fuel, saving in some other area - a small reduction in some other component, in exchange for doubling of mission outside time.
But despite all this, they decided not to.