Your question touches on several factors that influence the length of a mission:
- Mission preparation time
- Flyby versus orbital missions
- Transit time
Mission preparation time
You're trying to apply the rapid advances in mass-produced computing to other fields. Yes, if you wait five years, computers will be much faster. But Moore's Law is an exception: advances on most fields are much slower.
For instance, mirrors don't get cheaper if you wait five years, because it still takes the same amount of time and effort to fabricate them.
Here are a few factors that cause spacecraft design to take a long time:
- Space-rated components. Every part of the probe must be able to withstand the space environment: extreme temperatures and radiation mean that you can't use off-the-shelf components, everything has to be designed for space. For instance that ESA has developed its own microprocessor. To make sure all your components will work correctly, everything has to be tested thoroughly. This takes time. If you swap the microprocessor for a new, faster design, you have to start the tests all over again. The radiation also means you want your integrated circuits to use larger components, because they are less likely to be disrupted by radiation. This alone means you use chips that are several generations behind what is available commercially.
- The science instruments. These have to be developed specially for the mission. You can't just stick a commercial camera in a probe.
- Reliability. You have just one chance, the probe has to work perfectly when it arrives or all your work and money is down the drain. Cutting corners to save money would be counterproductive.
- Uniqueness. Each probe is a one-off, because each mission has very different requirements. So reuse is limited.
- Obsolescence. In my opinion, this isn't relevant. The question is, is the mission worthwhile with the level of technology we have now? If the answer is yes, then the mission should go ahead.
So on every level you're not just building the probe, but you're doing all the research and development that comes before. The only way to speed up the design phase, would be to simplify the design. See 'General observations' below.
Flyby versus orbital missions
Flyby missions may be cheaper, but they also severely limit the amount of useful science you can do. You're only near the planet for a couple of days, versus several years for an orbiting mission.
There's a lot of value in being able to observe a planet for a long time: weather patterns, seasons, influence of variations in the solar wind. An orbital mission also gives the opportunity to do high-resolution mapping of the entire planet.
The Pioneer and Voyager missions were successful. They vastly expanded our knowledge of the outer solar system. But they were followed up by orbital missions that were designed to 'zoom in' on phenomena only glimpsed briefly.
The launch is a significant fraction of the total cost. We're talking more than a hundred million dollars. Heavier launchers (to shorten the transit time) cost even more, so your simple probes are still going to be expensive to launch. The rational thing to do is spend more on instruments and less on the launcher. The same goes for slow trajectories using gravity assists. So what if it takes 8 years for the spacecraft to reach Jupiter? If it halves your launch cost, that means you get an extra mission out of your budget.
There is one political factor that comes into play: ESA prefers to use their own launcher for its space probes. At the moment, that means using the Ariane 5, with the limits on spacecraft weight that entails. If they wanted to shorten the transit time, they'd have to use an American Delta IV Heavy. The heaviest variant can launch about 2 tons more to a geostationary orbit (12 vs. 10 tons), but it'd mean paying \$150 million to an American company instead of benefiting the European space sector.
Using commercial electronics in space
There's been a lot of research into the feasibility of using commercial electronics in space (COTS). Most of this is focused on Low Earth orbit applications, where radiation is less of a problem than in outer space.
One study concluded that it's not as straightforward as hoped. A big problem is that the change cycle in commercial electronics is so short that by the time you've finished verifying that a part will work in space, the part will have been obsoleted and replaced. This makes it difficult to predict what the chance will be of the satellite still working after a year.
Another study concludes that even in LEO, many commercial parts fail due to radiation damage within a year.
Redundancy by itself is not enough; a single particle strike (single event upset, SEE) can change the behavior of a chip permanently (or temporarily). Commercial electronics don't allow you to detect this or route around it, so that means the first SEE can end the mission. This means you have to design the electronics to be fault-tolerant from the ground up.
A few years ago, an SEE corrupted Voyager 2's memory. Thanks to a fault-tolerant design, NASA was able to correct the error and resume normal operations. This problem could have ended the mission had commercial electronics been used.
NASA tried your approach. The Mars Scout program consisted of missions budgeted at \$480 million each. Phoenix and MAVEN were part of this program. NASA decided to end this, because it wanted to focus on surface missions (landers and rovers), and they wouldn't fit within this budget.
The Pathfinder rover was another low-cost mission, at \$260 million. It traveled about 100 m and was active for 80 days. Spirit and Opportunity cost \$400 million each. Spirit covered 7.7 km in 6 years, Opportunity is still active after 10 years and has covered 38 km so far, with lots more instruments than Pathfinder. In this case, the additional cost of the mission offered a more-than-proportional increase in the amount of science that could be done.
I think we can discount political disorganization on ESA's part. They're deliberately taking the long view, and have the political room to plan missions that take two decades. More so than NASA, it seems.
ESA missions are divided into 3 classes: Large (~€1B or \$1.2B), Medium (~€400M or \$480M), and Small (~€50M).
A final factor that makes the lead time seem very long:
While NASA typically selects missions after detailed design studies
and then launches in four to five years, ESA selects concepts far in
advance of launch and fills in the details after selection.