Our neighbours are a couple in their 90s who love their garden.
The boundary between us and them is a post and wire fence in very poor repair, that's been there 60+ years. There is however a thick hedge on their side of very prickly bushes. This has never been an issue, I trim my side of the hedge, the trim theirs, the boundary is adequate for us, we can't see the fence because it's deep in the middle of the hedge, neither have pets. When DC were small, neighbours used to poke lollipops through the hedge 🙂
Anyway, the time has come when they need to leave their big house and garden 
and apparently prospective buyers are asking for assurances that we will replace the fence (it is our fence).
I accept responsibility for the fence, although I'm happy with things as they are, if there was suddenly no hedge, I'd install a proper fence. However, putting a new fence in currently would be a huge job....because of the hedge.
What's a suitable response? Neighbours are lovely and I certainly don't want to cause them any stress and I'd like to be on good terms with any new neighbours, but...