26 years (it's still going after all because of the repairs done by shuttle missions) is a long life for a space telescope and particularly for one during it's time. Before Hubble most telescopes were only around for a handful of years before being shutdown.
There's a big difference between designing to fail and designing to be repairable. Even if it's possible it could have been cheaper to launch multiples but it's unlikely NASA could have gotten the funding to start making Hubble 2 in time for it to replace Hubble.
Granted, nobody died, NASA got great press from the Hubble, and it made the shuttle seem useful. And sure funding would have been harder due to less feel good missions, but easier from 1+ billion lower costs. So, I am not going to call it the dumbest thing NASA has done.
However, as to lifespan, from April 24, 1990 to Dec, 1993 the Hubble was significantly 'impaired'. If the next two averaged 10-12 years and the original continued to be useful for a few years that's well past break even. Especially considering the Hubble always suffered from a defective mirror even after the first servicing mission help that's not unreasonable.
PS: By designing to fail I don't mean they intentionally made it worse, just assuming servicing caused a wide range of problems. Sure, NASA did learn something from going though this exercise. But the highly political nature of the organization limits such benefits.
There's a big difference between designing to fail and designing to be repairable. Even if it's possible it could have been cheaper to launch multiples but it's unlikely NASA could have gotten the funding to start making Hubble 2 in time for it to replace Hubble.