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"though many factors could ultimately affect Webb’s duration of operation." JWST also needs helium coolant with no possibility to recharge therefore limiting its lifespan to ~10 years, I believe. Interesting about Herschel and L2.

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It actually uses a closed system:

https://webb.nasa.gov/content/about/innovations/cryocooler.h...

Doesn't liquid helium always evaporate?

IIRC, the helium coolant is only used on one of JWST's 4 instruments. So if they run out of helium the other three can still be used.

It does use helium only as refrigeration fluid in closed cycle, it does not get depleted. Building a cryocooler capable of achieving so low temperatures without boiling helium off was a major hurdle in JWST development.

Isn’t helium notoriously hard to contain without leakage even in a closed system?

that's hydrogen. IIRC from my chemistry days, Helium is hard to contain in plastic closed systems, but "generally OK" in metal tubing.

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