Attitudes towards External/ Political Motivations for Technical PhD
My concern is, if I interview with potential future advisors with this kind of story, even if the political problem I'm focused on has a long, 1-2 decades timeline for resolution, a future advisor might find this kind of attitude &quot;disloyal&quot; to a field or otherwise unacceptable.

Importantly a PhD in a given field really is just a piece of paper signalling you are capable of independent and novel research. It's sort of a baseline expectation for a researcher and nothing more. Kind of similar to how your BSc simply qualifies you to speak intelligently about broad topics in your field.
I'm not aware, personally, if any situation where switching fields/leaving a field would be seen as a negative. In my field (Computer Science) it's not uncommon at all to see researchers publishing in broadly related fields such as math, engineering, economics, or really any other place Computer Science is used. Though, to be fair many more stick to their specialization.

This model of thinking about one's career, &quot;If I'm successful the problem will be solved and I can quit my job&quot; contrasts the life-long academic model. At the same time, I feel that's how many people are motivated to enter research, not from the perspective of &quot;I'm interested in X technology&quot; but &quot;I want to solve Y problem&quot;.

10-20 years is more-or-less a career (using your numbers above). I don't think anyone would look at you negatively for quitting at that point.

&quot;I'm primarily motivated by recent, specific political circumstances&quot; may not sound very dedicated coming from a student entering a new field for them, even if all signs point to this problem staying around for decades. And for certain issues, like defense/ security, advisors may object to training students with this motivation because of their ethical views.

Political motivations really depend on your field. I can only speak for my field but if it wasn't for a need for better computing technology in WW2 the field of Computer Science may not be what it is today!
However, importantly your political motivations must necessarily align with your advisor's. If your advisor does not see your problem as important it will likely be disregarded. Your goal during your PhD is to learn how to do research under the guidance of someone with significant experience. It's approximately equivalent to a journeyman position in labor but for academia. Your advisor is there to help you find topics that will get you out of your PhD on time and hold your interest for 4-6 years. If you get a read from your advisor that what you want to do is ethically/politically contrary to their beliefs you can either accept this and do research with the end goal of contributing how you want after your PhD, or attempt to find a new advisor. Your PhD is not your entire career. If you get your PhD you'll have plenty of time to do work in things you enjoy.
In general your post gives off a small sense of superiority. What will sink you is this sense you seem to have that you will solve a major problem in a field. It's an honorable thing to think about but to use a direct example - if I told my advisor my goal is to solve P =? NP in 20 years I would probably be told to seriously evaluate whether I understand what research actually is.
Keep this in mind when broaching this question. It's good to have aspirations of greatness but remember to keep them in check. Research is incremental. Reframing your thoughts, and the way you broach the question to advisors, into something like &quot;I am extremely passionate about XYZ and I want to advance the body of knowledge in this field during my career&quot; will both make you look humble and intelligent.