And a 9 years mission that breaks after 8.5 years is a mayor failure that wasted millions of taxpayer's money. A 2 years mission that last 8.5 years is a successful result of good engineering practices.
Probably the Helium evaporation is too easy to simulate and they only put there the right amount plus 25% just in case. The problem is that Helium is a consumable, so the tank has a finite duration. You can't try to continue until something breaks.
Does anyone know why the Herschel was so far from earth? Easier stationkeeping at L2?
So it's always in the sun, which means it has continuous heating, and thus reaches thermal stability, and doesn't expand/contract/whatever as it passes in and out of shade.
It will be in a certain kind of orbit around the L2 point, not right on top of the L2 point. This means it is not in Earth's shadow, as you note. It's in its own shadow. But if it were in a conventional orbit centered around Earth, it would have thermal issues, observational issues, etc., because pointing to keep the telescope away from the sun would be a big problem.
My friend across the hall uses data from Planck, which is at L2, and I used to use data from SoHO (before its camera was shut down), which is at L1. We used to joke around about how each other's instrument was in a too cold/too hot place.
Here's a nice illustration of Planck's orbit. (Planck was the sister satellite to Herschel.) http://planck.cf.ac.uk/mission/orbit
The cost of a SpaceX launch to LEO, 85 million dollars. So we launch three Spacex cargos for a tanker, a refueling facility, and a pre-fueled space craft. And for 250 million dollars we get a fully re-fueled telescope. How cool would that be? Since its in the L2 point it will wait for us to get to it without de-orbiting.
 http://esamultimedia.esa.int/docs/herschel/Herschel-Factshee... - 1100 british pounds for total cost of instrument and mission, if half is the cost of the instrument that is 550 million pounds or about 852 million dollars.
Sometimes experiments get handed off as training tools for new aerospace students. When Gravity Probe B ran out of helium, control was transferred to the US Air Force Academy for navigation experiments 
It's sad to see good experiments go, but with finite resources, sometimes the young must take precedence over the old.
Edit: Looks like there are alternative repurposing plans, perhaps , but a bummer of a use for that mirror.
"The instruments are cooled with over 2,300 litres liquid helium, boiling away in a near vacuum at a temperature of approximately 1.4 K (−272 °C). The 2,300-litre supply of helium on board the spacecraft was a fundamental limit to the operational lifetime of the space observatory; it was originally expected to be operational for at least three years."
I hope James Webb will get some extra lifetime as well.
When you get this close to absolute zero active refrigeration systems are no longer effective, but the phase change of liquid helium to gas still manages to extract a little bit of energy from the system cooling it down just that much further.
But I believe that Planck was the first satellite to cool to below 1 K using only cryocoolers. Planck actually uses a dilution fridge to get to 100 mK, which is kind-of astounding.
I don't know why Hershel used liquid helium instead of a cryocooler, but my guess is that the technology for cooling with liquid helium in space is very well understood and reliable, so it's a risk thing. That's not to say that nothing can go wrong; there was a japanese telescope that lost all its helium within some very short period of time thanks to an engineering mistake.
This is a nice whitepaper about cryocooling in space: http://cmbpol.uchicago.edu/depot/pdf/white-paper_w-holmes.pd...
The cryocooler technology (say, to a few kelvins) has proved hard to get ready for space. For example, out of the 10 or so technologies that were judged most risky for JWST, the 6 K cryocooler for the MIRI instrument was the last to be judged ready for space ("at TRL 6" in the jargon) (http://www.stsci.edu/jwst/news/2007/jwst-passes-tnar).
Despite being judged ready, the JWST cryocooler has proved very challenging to build. The effort now has frequent reviews with the director of JPL (and a high-level counterpart at NGST), and tens of engineers are now working on the system.
Part of the problem, as I understand it, is that the heat has to be taken away and radiated at a site distant from the IR detector. This requires a large structure, and a deployable radiator. This large structure can't leak much heat back into the spacecraft bus or instruments, and must not be disturbed by the vibrations of launch. Additionally, vibrations of the cryocooler must not affect the telescope optics (2 micron resolution).
You can tell that these requirements are fundamentally opposed to each other ("be large, don't vibrate, don't touch anything else").
Then you either get half the mission data, or extend the mission by a factor of two, which means twice the operational labor cost, plus every other part of the craft has to be double lifetime rated, which could get expensive. Hubble used to burn thru gyros on a regular basis, so now you need them to last twice as long or launch with twice as many spares. Or maneuvering propellant if any, now you need twice as much for station keeping.
You can easily get painted into a corner where the cheapest way to run a mission twice as long is to launch two of them. At that point you're better off saying "you know the cryocool half the time and take data half the time idea? Yeah scrap that idea"
This is before we started on energy issues. A heavy helium tank doesn't use much energy. But cryocoolers on earth take quite a bit indeed, well, at least compared to a couple watt transmitter and all that. Whats heavier, a tank big enough to last mission lifetime, or a cooler and a stunning array of solar panels to run the cooler? Or since it only runs half the time as per above to prevent vibration issues, you could put a battery in which is heavy and becomes another exciting point of failure. This makes the existing power system more complicated and less reliable possibly shortening the theoretical craft lifetime to less than you'd get if you just launched a big simple tank.
Yes I know there are good reasons why they put it at a L point, but unscheduled unplanned maint is exactly what the ISS could have been perfect for. Position this dude in the same orbit but at a phase 10 miles away so its not too close, and send a live astronaut with a new battery or whatever anytime "something" fails. To say the ISS project is not managed this way would be an understatement. But someday, an unscheduled repair shop in space will exist, and probably be quite profitable.
That Planck figure is extremely impressive, mind-blowing really. That's colder than the space surrounding it.
It is good for great experiments to have an end.