"It took two years to get to Jupiter.
30+ years later, it's still not hit one light-DAY away from us.
Sure, it's not got a huge propulsion, but it makes you wonder how you intend to keep something powered and propulsive for decades or even centuries."
But bear in mind it has 16 hydrazine thrusters and it's mission profile called for it to visit Jupiter and Saturn before cruising out of the solar system. The remaining fuel will be carefully rationed on maintaining it's attitude and pointing capability so that it can maintain comms with Earth until it's electrical power runs out - not frittered away on thrust such that the craft has electrical power but has no fuel to orient itself for transmission.
If your goal was inter-stellar travel, then consider that reaching 1% of c requires 85 hours at 1g. That would have you at 3000km/s - far faster than Voyager's 17km/s.
Heck, 1 day at 0.5g would get you to 423km/s, which makes Voyager's 17km/s look decidedly sedate.
Voyager 1 is not going quickly because it's a scientific mission who's job is to coast and see what it sees.
If you actually want to get somewhere, you could do it much, much faster.
The issue is less with fuelling it for decades as much as simply providing enough fuel mass to sustain the necessary acceleration burn (and corresponding deceleration burn at the far end).
This could certainly be done with low-mass craft like Voyager using existing high-specific-impulse ion technology.
It becomes more expensive and difficult when you scale to include life support for organics, etc. As you need a lot of power and a lot of fuel to accelerate those additional kilograms. A couple of planetary slingshots on the way out could help maximise speed relative to fuel consumption, but it wouldn't need to take decades to leave the solar system if you were in a hurry.
It doesn't change the fact however, that space is really, really big, and it would certainly take decades - if not centuries - to do 8 light years to our next nearest star (depending on the craft and mission profile). But not the thousands of millenia it would take Voyager 1.