That makes absolutely no sense. If you bought it for income replacement, and you already covered your income years, you're gonna be upset that you didn't gamble on your health going bad?
Based on that logic, one should buy decreasing term, or should reduce the face amount every year (or every 5 years) commensurate with remaining earning capacity.
One thing I can assure you, that $7 more is commensurate with the additional risk and value to both sides. It's not a 'deal'. Therefore, you have to ask whether that coverage is needed or even wanted. If you can truly convince the client that their coverage needs extend past age 65, it's straight up negligence to push the convertible 15 year instead of a 20 or even 30 year term.
You might have a crystal ball, or better yet be a נביא. I do not have one, and am definitely no נביא! I don't know what will happen to me in 15 years, let alone someone else. Why don't you survey some 59 year olds, and see how many of them that are positive they only want (I'm not even going to use the term need) the coverage for the next 15 years, would be willing to pay another $7 a year (that's less than 2 cents a day) for the remote chance that there might be a reason for them to want it beyond 15 years.