>The project was not really designed to reach interstellar space originally.

Not only that, Voyager 2's flyby of Uranus and Neptune in the late 1980's was originally not intended either. As an aside, to this day Voyager 2 remains the only spacecraft to ever have visited either planet, and there are no firm plans for a followup, just some loose ideas about maybe launching something in the mid 2030's. Anyway, doing the Uranus/Neptune part of the mission required extensive software upgrades, which introduced Reed-Solomon error correction and image compression capabilities, among other things - the software as launched would not have been capable of a meaningful mission to Uranus and Neptune.

These days the Voyager program is lauded as an astonishing feat of engineering and one of the most inspiring science and engineering achievements of all time, but in the early 1970's the entire idea was NASA's red-headed stepchild and ended up cut down to a bare minimum. The Grand Tour mission concept (taking advantage of the extremely rare opportunity to visit Jupiter, Saturn, Uranus and Neptune in a single mission) was pitched as early as 1965, and by the early 1970's there were plans for launching four spacecraft, two bound for Jupiter-Saturn-Pluto and two bound for Jupiter-Uranus-Neptune. These were referred to as TOPS, Thermoelectric Outer Planets Spacecraft. But then people started complaining that it might cost a billion dollars (Apollo had cost $25 billion) and the whole thing became intensely political. Quoting from Voyager: The Grand Tour
of Big Science (https://www.nasa.gov/history/SP-4219/Chapter11.html) by Andrew J. Butrica:

> Further complicating matters was Senator Clinton P. Anderson (D-NM), champion of the Los Alamos nuclear weapons laboratories and an enthusiast, until his retirement in 1973, of the development of a nuclear rocket engine called NERVA. As chair of both the Senate Aeronautical and Space Sciences Committee and the joint Atomic Energy Committee, Anderson provided NASA and the Atomic Energy Commission over $1.4 billion, about $500 million of which was spent in Los Alamos, for the development of the NERVA engine, which, Anderson held, was ideally suited for exploration of the outer planets, as well as for more advanced missions. Anderson worried that NASA and the OMB were shifting money from NERVA to fund Grand Tour. When the NASA budget came before Anderson's Aeronautical and Space Sciences Committee on May 12, 1971, his committee voted five to two to reduce Grand Tour's budget, while an amendment to increase NERVA funding passed. Werner von Braun worried that ardent congressional interest in NERVA would force a loss of Grand Tour in favor of a NERVA that had "no place to go."

> Meanwhile, NASA was trying to include Grand Tour as a new start in its 1972 fiscal budget. The Friedman report moved the Office of Management and Budget (OMB), in March 1971, to ask NASA to study simpler, less costly spacecraft alternatives to TOPS. The OMB also attempted to delay the Grand Tour start-up to fiscal 1973.

> (...)

> As NASA prepared its fiscal 1973 budget, rumors spread that the "budget pinch" was going to affect planetary programs deeply and that the reduction of the Grand Tour payload from 205 to 130 pounds was "a likely fact of life." Furthermore, Grand Tour now began to compete for funding with the latest NASA human program: the Space Shuttle. The fiscal 1973 budget request NASA submitted to the OMB on September 30, 1971 included both Grand Tour and the Space Shuttle. Throughout the autumn of 1971, several press reports presciently reported Grand Tour's vulnerability to a possible elimination or reduction. On December 11, 1971, James Fletcher, NASA administrator since April 27, 1971, learned from White House officials that Nixon was prepared to approve the shuttle program and that Nixon would not let NASA simultaneously fund the shuttle and the full TOPS Grand Tour in the 1973 budget or in subsequent fiscal years. Fletcher had to decide which was more important: Grand Tour or human flight.

Fletcher chose the shuttle, and what could be squeezed into the budget was an extension of the Mariner program to visit Jupiter and Saturn only. For budget reasons the spacecraft development was kept in-house at JPL rather than contracted out, and at JPL the dream of the full Grand Tour was still alive:

> Despite the limited aim of the Mariner Jupiter-Saturn, the mission had the Grand Tour launch window, that rare planetary alignment, and the engineers at JPL still had every intention of building a spacecraft that would last long enough to visit Uranus and Neptune. This intention was not emphasized; however, it was stated that a Mariner Jupiter-Saturn spacecraft might continue to Uranus if its mission at Saturn proved successful. The scientists working on the project knew that Mariner Jupiter-Saturn was going to go to Uranus and Neptune, too. As Bradford Smith, Leader of the Imaging Team, explained: "We understood at the time the enormous potential of this mission, that it could very well be one of the truly outstanding if not the most outstanding mission in the whole planetary exploration program."

Also for budget reasons, the spacecraft were limited to mostly reusing existing technology. Getting reprogrammable computers (without which they could never have been kept alive in the way they have) required a separate budget grant from Congress:

> Despite the reliance on extant technology, some money was set aside to develop new technology. Congress and the OMB approved an additional $7 million to the Mariner Jupiter-Saturn appropriation for scientific and technological enhancements. Part of that appropriation went to develop a reprogrammable onboard computer, which proved vital to maintaining Voyager 2 as a functioning observatory in space. Without properly functioning hardware, no science could be conducted.

In the end only Voyager 2 was launched on the full Grand Tour trajectory that would allow visiting all of Jupiter, Saturn, Uranus and Neptune; Voyager 1 was launched on an easier and much faster trajectory that would take it only to Jupiter and Saturn. Even then, the official decision to extend the Voyager 2 mission to Uranus was only approved in 1980.

Human spaceflight and its enormous appetite for money has always been a huge threat to actually exploring the solar system beyond Earth orbit, and we should be very glad we got even the very diminished Voyager program that exists today.