Hi and welcome
What a wonderful age to live to and what a credit to your good care!
I've been faced with the same situation earlier this year when 8 years old Morwenna survived both her closely bonded litter sisters. I do have other guinea pigs but she was not interested in them; not even in my second unbondable 6 years old widow. Their interaction through the bars consisted in mutual teeth chattering. Morwenna followed her last sister a few months later in June.
Unless you want to continue with guinea pigs, in which case companionship through the bars would provide stimulation (even if it is just a 'stay away from me'), many really old sows generally prefer their usual surroundings and daily routine over any changes and companionship. You may have to review if a creeping depression sets in. A lot depends on your last standing girl's personality.
But I have also adopted the last standing member of a group at 7 years old when she became very withdrawn in the wake of the loss and her busy owner (a nurse working 12 hour shifts) felt she couldn't be with Calli as much as Calli needed. It took about 10 months of living next to each other for my widowed rebel Tegan and Calli to consent sharing a cage. But as the spark between them has always been there, I did persevere. Calli lives to celebrate her 9th birthday thanks to having found new company, and 6 years old Tegan had learned the valuable lesson that companionship was more important to her than coming out on top all the time. She genuinely missed Calli.
You may find the links below helpful; they address the various practical challenges you can face with single guinea pigs in a range of situations (including bereavement) and how to spot signs of depression past the initial mourning as well as how transferring their companionship needs onto you can manifest. What you can practically do, very much depends on your local options, unfortunately, so there is not a clear one solution fits all cases.
Single Guinea Pigs - Challenges and Responsibilities
Looking After A Bereaved Guinea Pig
Moody Guinea Pigs: Depression, Bullying, Aggression, Stress, Fear and Antisocial Behaviour
You may also find this very practical but sensitive guide about caring for the elderly and frail and also about support for yourself when you are confronted with the challenges that come with an end of life care situation: co.uk/threads/caring-for-older-piggies-and-facing-the-end-a-practical-and-supportive-information-collection.190997/
It is never easy; especially not after all that time but the links will hopefully make it easier for you in charting your own way of doing what is best for your own situation and tackling common problems associated with that.