How do you deal with paying off debt as a tangible goal when the clients only have a mortgage at a low interest rate and it doesn't make financial sense to pay it off ahead of schedule?

As your client's financial advisor it's your job to give them the best advice. If the best advice is to keep the mortgage then give them that advice. There are also emotional considerations for any goal and debt, especially, can have some powerful emotional motivations. If you have a client who despises having any debt and paying it off will not inhibit the achievement of their other goals then the emotional benefit to the client may outweigh the technical "financial sense" of keeping the low interest debt.