[gadnuk]

You are right. The source for my statement above is this link: https://www.americanscientist.org/article/jwsts-limiting-fac...

At the end of the link is the clarification:

Drs. Heng and Winn respond:

As pointed out to us by Drs. Jason Kalirai and Jason Tumlinson at the Space Telescope Science Institute (STScI), as well as Mr. Sykes, our article misstated the reason for the finite lifetime of the upcoming James Webb Space Telescope. The mission duration of 5.5 to 10 years is not limited by the supply of liquid helium, as we stated. Rather, it is limited by the supply of hydrazine fuel needed to maintain the spacecraft’s orbit.

Thanks for the correction, will edit my parent reply.

[skyfaller]

Does this mean an ion thruster or solar sail could have significantly increased the service life? Or would something else give out shortly after the fuel runs out?

[Vvector]

It is due to orbit in L2, in eternal shade of earth. So no solar power

[gadnuk]

No, that's not true. It will be orbiting around L2 and not stay at L2, so it will have access to sunlight, which powers the solar array that faces the sun. The actual observatory and the mirror are shielded by the sunshield.

Here is Dr. John Mather explaining it: https://youtu.be/4P8fKd0IVOs?t=1321