There is a book called the grand tour about the coyager program. I cannot find the exact quote, but they couldn't get funding to design the mission they wanted to do the grand tour; they could only get funding for Jupiter and Saturn. They developed a mantra of something like "don't plan for a long mission, but also don't make any decision based on a short mission life".

I suspect we see a similar process play out with nearly every probe mission. They couldn't get funding to build something that'll work with a REALLY HIGH probably of working for a year, but they made trade offs to get them a high probably of it working for a year.