Using your last few years to exercise your brain and ward off cognitive decline might be the best way to ensure those last few years are fulfilling and not just marking time before the end.
Working on Colorforth might be the greatest meaning in his life.
He does not think working on Colorforth is worth it anymore, so it could actually be detrimental to do so.
It's hard to imagine an extremely niche software tool to be the greatest meaning in someone's life.
My Dad is like this. I'm like this. My son is like this.
Unless we're busy, pushing ourselves to build something, fix something or just outside doing something we don't feel the reward.
My Dad told his motto, "A rolling rock gathers no moss - until it finally stops rolling." He told me that in his 50's - he's in his 80's still out in the garage refinishing old furniture and giving it away. The drive the man has just never burns out.
That's completely up to him, and if that's what he wants to do, then that's the best use. No one can say what is best for anyone else.