Not that I disagree with you that building cost efficient instruments would be a good thing, but I think you are underestimating the difficulty of refilling Herschel with liquid Helium. Liquid Helium cryostats are difficult enough to work with on earth (see: LHC), doing it in space seems like a really daunting task. I also expect that adding the option to refill would greatly increase the complexity of the design of Herschel, making it more expensive and offsetting the gains you would get from the increased longevity.