Well, the effect of all of the books is that they only have one another and, at a safe distance, their adult children, who have walk-on parts as dispensers of money for train tickets, advice (when their daughter says she's going to buy them a new mobile that holds its charge better so they can phone her daily from the path and Raynor complains she's acting like her mum) or dogsitting for Monty the dog. (Which, in fairness, may be at the behest of their children. Many people don't want to feature in family members' books.)
The message of all of the books is that this is the way to be properly married, not having low-voiced fights in supermarkets at the weekend and veging out watching TV -- you should walk off into the sunset together, with nothing but your burning love and wild nature to sustain you.
I assumed when I first read TSP that their farm was actually remote, so, if they had spent 20 years restoring it, farming it themselves, bringing up two children, and running a holiday let, while doing other odd jobs to top up a farm income, they had probably become isolated from the rest of the world and withdrawn a bit from friends and extended family.
But now it seems that it was only really a smallholding/hobby farm, and not particularly remote, and that they had family close by, I'm assuming it's just poetic licence that suggests it's just the two of them against the world.