Steve, I would not expect this koi to keep on improving. If it does it well be very little. But what do I know? Not much that is for sure.
Thanks for being patient with this one (5years) and is not uncommon for this slow development to occur. As in other varieties as well. Most do not go through this to understand how koi can change. Most are in it for the instant gratification. And not the enjoyment of the process of seeing the slow development. And that is ok. Were all different.
It's a gamble. Waiting for the slow development and the ability to understand that a koi might have reached it's peak is a interesting thing indeed. So it comes down to do you keep it for a few more years or not? That is the tricky part and part of the fun with koi.
Just a small part of the koi hobby. I have seen koi that were said to develop in the future by experts that failed. And I have seen many others that did develop well. It's always a roll the dice. And just a part of the hobbie.
I think we all enjoy different aspects of the hobbie more at different levels of koi understanding.
I am to the point of just liking the colors.
All Koi are a gamble but I have had a theory that it is the strength of the Beni that determines the clarity of the Beni plate in the end. The fact that the Beni was thin, and the Sumi showed through at an early age, was not the determining factor on what it would become. These Goshiki were very inexpensive, and were never going to be truly amazing Koi, but they could be inexpensive learning tools.
How do you know how long to keep a Koi? This is a difficult question to answer but, I would only do it, as long as there was some improvement. From the beginning you could see some improvement in the progression pictures.