10 years was not quite worst case but definitely a conservative estimate. L2 is an unstable orbit and needs constant corrections to not drift away, so JWST has

10 years was not quite worst case but definitely a conservative estimate. L2 is an unstable orbit and needs constant corrections to not drift away, so JWST has much more significant fuel needs than if it were in say Hubble's orbit. How much fuel it would have left for these maneuvers depended on how precise the launch was (i.e. how much fuel it needed to spend on corrective burns just to get to L2). This was obviously uncertain until after the launch, but ten years was a lower bound above which the launch would not be considered a major issue--if it only had say 5 years of fuel left after the launch that would be an indication that the launch was flubbed some how. TLDR: JWST was not "supposed" to last only 10 years. It was designed and built to last much longer, that was just a lower bound assuming nothing went very wrong. cf. Mars rovers operating long past the "planned" mission length, e.g. Opportunity's "planned" 90-day mission turning into over 5,000 days. It is vastly unfair to the people who designed and built Opportunity to say it was "supposed" to last only 90 days. It was built to last much longer; 90 days was a lower bound for the mission to be considered a success.