The article you linked says absolutely nothing about the helium cooling medium.

Three of the four imagers on the telescope are passively cooled and will work as long as they don't succumb to radiation, diffusion, etc. The fourth one (MIRI) has a cryocooler that uses liquid helium, but it will leak out very slowly and mechanical wear and electronics lifespan is expected to be the limiting factor there. [0, 1]

As stated in other comments, the primary driver of lifespan is a combination of how stable the telescope orbit is, and the resulting amount of fuel needed to keep the telescope in a stable orbit. Depending on how things go it has enough fuel for somewhere between 5.5 and 40 years of operation. Assuming nothing else goes wrong. :)

"Webb is designed to have a mission lifetime of not less than 5-1/2 years after launch, with the goal of having a lifetime greater than 10 years." [2]

0: https://jwst.nasa.gov/content/about/innovations/cryocooler.h... 1: https://www.nasa.gov/feature/jpl/how-cold-can-you-go-cooler-... 2: https://jwst.nasa.gov/content/about/faqs/faq.html

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.

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?

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

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

I’m sure one of our manned moon missions can swing by and top her off

/kidding

//a little