Not to downplay how impressive the Voyager probes are, but it seems they packed a fair bit of hydrazine. From "Engineering the Voyager Uranus mission":
While it was not a design requirement, the option for an extended mission past Saturn was always protected, unless it meant compromising a major mission objective at Jupiter or Saturn.
Even though the probability of Voyager 2 lasting another five years was calculated to be in the range of 60 to 70 percent -- well below NASA's usual acceptable probability-of-success threshold -- the decision was made to send Voyager on to Uranus.
After its Uranus encounter, Voyager 2 still carried 48% of the hydrazine
initially loaded into its tanks, eight-and-a-half years before.
[1]: doi:10.1016/0094-5765(87)90096-8 (can be found on the hub of science)
While it was not a design requirement, the option for an extended mission past Saturn was always protected, unless it meant compromising a major mission objective at Jupiter or Saturn.
Even though the probability of Voyager 2 lasting another five years was calculated to be in the range of 60 to 70 percent -- well below NASA's usual acceptable probability-of-success threshold -- the decision was made to send Voyager on to Uranus.
After its Uranus encounter, Voyager 2 still carried 48% of the hydrazine initially loaded into its tanks, eight-and-a-half years before.
[1]: doi:10.1016/0094-5765(87)90096-8 (can be found on the hub of science)