I take care of dogs for a living, and I currently have clients (an older couple...75 and 79 with multiple health issues), who are trying to adopt a third dog (Shih Tzu) and are having a difficult time getting a rescue group to place a dog with them. They have two shih tzus now and take good care of them. The oldest is undergoing chemo for lymphoma and I know they are anticipating his loss...and would like their other dog to have a companion. But...I also know that their only child does not want any dogs that they may leave behind. And if one of them goes, the other will have tough time caring for two or three dogs alone. So I hope they continue to hit road blocks and just settle with the dogs that they have. So I think family opinion plays a big role, especially if a family member may be responsible for taking on the dog...or finding a home in the event that you pass or become incapacitated.
I've also seen older people get puppies and they'll go with the breed that they love...like a golden for example. And the dog is just too big for them...etc. My next door neighbor adopted a dog over a decade ago that has worked out fine, but he's a big dog, which is what she prefers. She's now in her 70s and talks about adopting when he passes and would prefer another big dog. If asked, I would strongly encourage her to go with a smaller rescue if she could even qualify for one. I may even butt in on that decision, if it happens, as I know her really well. I'd worry about her getting hurt, pulled down at this point.
As for DH and me...we currently have 3 dogs, two within 2-3 years of the end of the life. From that point on, we'll have just one small dog that we can travel with. And in our 70s, depending on health, we'd make sure the dog had a home if we passed or became unable to care for the dog. We're 55 and 56 now...and our youngest schnauzer will take us into our early 60s. It's hard for me to imagine not having a dog, but we also might enjoy the complete freedom of travel without a dog for awhile. We can volunteer for various dog rescue groups as we travel to fill that need.