You can live on coconuts for the rest of your life. They contain all the important nutrients a body needs. The only thing that could decrease over time would be your eyesight or the health of your tooths, which is why you shouldn’t rely on coconuts alone.
That is really a typical scenario. You are stranded on an island with nothing but coconuts available. In this scenario you actually got lucky. Coconuts are very healthy and will help you keep going for a very, very long time. If you have enough coconuts, you don’t even need to drink water.
Coconuts contain vitamin C as well as sufficient fructose, fats and vitamins. Iron, calcium and magnesium are also part of the coconut; the list goes a lot further. In many countries the coconut is worshiped as a remedy for everything. However, if you do not brush your teeth often enough, you could get tooth decay, which would be very impractical for further consumption of coconuts or of course for further stay on the island.
So it’s enough to survive healthy on itself, but it is definitely not the best of the best, even when it is definitely in the top league when it comes to these kind of scenarios. In the long term, you should start fishing and turning water into drinking water on an island in order to protect your teeth.
So the bottom line is, with the pure consumption of coconuts you would not be in a bad way on a lonely island, but occasionally look around for a change, fish a few fish from the sea, or look for other fruits that you already know . Otherwise one thing could just happen to you after a certain amount of time: Coconuts don’t taste anymore.