This is an important and well-thought-out post. I know of more than one person in this situation.
1) I've noticed over the years that most poly people do develop primary life partnerships as they age, usually of the "open marriage" variety. Solo networking poly seems to be mostly done by the young and early middle aged.
2) Simply by numbers, the problem will be self-solving. If there come to be more than a few people in this situation -- aging, poly, wanting a permanent life partner, and actively looking for same -- they will find each other.
3) The sad reality, working against point #2, is that some people who want to find a life partner cannot for good reason: serious problems with personality, behavior, or health that are too much for others to cope with. This could be anything from uncontrolled mental illness to something as simple as poor hygiene (a weakened sense of smell is common with age) and no one advising them that they need to use a scrub brush when showering, or clean their teeth and tongue properly before breathing.
-- Alan M. (age 62)