Thank you for all that information. That fits with my gut feeling! But yes, they can be offered the opportunity in time.
If future frailty led to preference for companionship over status, how would you know they're accepting the situation rather than too weak to express unhappiness? Or am I over thinking it..
You are overthinking it. Even in old age, they are still perfectly able to express their feelings and opinions and to signal very clearly whether they want to be together or not and reaffirm that commitment in between the rounds of working out the leader and all the dominance. They may be arthritic but they are not necessarily dying as you seem to imagine.
I have rebonded a number of 7 and even 8 years olds with both other oldies (often old companions or long term neighbours where I could feel a spark between them) or babies that could not challenge their authority even when they were already frail when there was a younger companion who needed their own long term interests taken care of themselves.
But if there is one thing that I have learned in terms of match-making, it is to not get my hopes up too high. Take it from the perspective that your piggy is in full control of how they want to live their life - that is a very precious gift you make them.
Younger piggies rely on companionship much more and no piggy is ever too old for companionship but that doesn't mean that all piggies want to give up their privilege and status - or are able to jump over their fear of others - for the sake of companionship. Piggies are every bit as complex has humans when it comes to relationships and when you really give them the freedom to express their own preferences. It can be frustrating for you as an owner when they refuse to nicely tick your boxes - but those boxes are, after all, still human boxes (even if of a better standard!)
Knowing that you are allowing your piggies the full freedom and control over their own life choices (even if they it doesn't make for a life of unblemished bliss) may be a somewhat thorny path but it does bring you ultimately a level of satisfaction that you can only reach when you take your pets totally seriously as full individual personalities, however small they are in body, and put their wishes before your own desire of a neat happy ending at all times.