OP - being blunt, you're not really selling yourself here. Your posts have been about why you cannot do something (age, perception, closed shop ... ) and there is little about what you actually can do. If I was reading this and happened to need to recruit someone with your experience (and there may be someone browsing MN in exactly that position) then you are not exactly leaping off the screen as being the ideal candidate.
You're probably right - you might never work for someone else in your profession for all of the reasons that you list. Too old (for products aimed at "youngsters"), too experienced and therefore a threat to younger managers, too expensive etc.
So how about a change of approach to being the Product Manager for an excellent, proven product - YOU!
Everything that you list as a negative should actually be a positive. You have x years experience in the industry. You have a PhD? You have worked in sales, marketing, research, new product development, PR, production. You've been there - done that, so you're not going to be fazed by new challenges. You presumably have a detailed knowledge of how the industry works - and probably who the major players are - so start utilising this knowledge and experience. If you cannot find a company to employ you, then you'll have to employ yourself - and you already have experience of freelance/consultancy work.
I know nothing about your industry, so I'll use a different one to illustrate the point. Imagine you had 20 years experience with "Cola Drinks Inc" - a huge multinational company where you worked in sales, marketing, product development etc. before being made redundant. Which other companies could use your knowledge?
Anyone starting up a new drinks company might need someone who knows about product development, packaging, hygiene requirements, product approval and certification, production options, marketing etc. If they want to be stocked by Tesco and Sainsbury then they need to understand how these supermarkets operate - even just how to get an appointment with a buyer. Would they be better selling through small specialist retailers, or to restaurants and hotels? What about exports ...? What about sourcing raw materials, briefing ad agencies, on-line marketing ... etc. What about finance and budgets ...
Someone with this knowledge could also advise other companies in other industry segments - the knowledge could be transferred to any company in food and drink (a fish company, sausages, gourmet soups, jam, micro-brewery beer or spirits) or any company wanting to sell retail products. Some lateral thinking can reveal many more opportunities.
As for being 55 - it's the ideal age. As an external consultant you are not a threat to the management - you should be seen as more of a sounding board for ideas or as a sparring partner.
You have a friend who is in PR - so get together and start putting together "Brand LipstickLou". Then you begin calling all of your contacts in your industry - for example there will be companies that don't want to employ a full-time person but who could really use someone for a short-term project or some expert input a couple of days a month. Read the industry trade papers and websites. Even launch yourself through those media. When things open up again, get to the industry exhibitions and trade shows.