I must say I love Graig's work and have bid on some in the past that have arose and will continue or possibly commission a work. This question seems like a bit of trap as people's opinion of art are often like religious beliefs and unwavering.
Okay this is a book and I know that. I started typing a reply and this is as brief of an answer I have to such an open question.
That said, as an art collector, collectibility is hard to gauge and completely misunderstood by the general public. There is a general confusion caused even by the flow of a traditional museum. I will try to outline in a very general way, keep in mind a true even general outline would be a several hour lecture.
As a visitor you will enter generally in a medieval/renaissance area with biblical or historical subjects these are beautiful studies of light and color. Next will be baroque/rococo, this continues the flow of traditional realism and romantic beauty. We move on to neoclassical which are generally historical scenes and beautiful. These will be the roman historical paintings done in the early 1700s to early 1800s.
What neoclassical signifies is important, as you would have noticed in the museum at this point is the majority of the art has been very focused in realism and technique. Up to the neoclassical, the flow has been the photography of its time. Telling a story of history or in many cases, showing current events, people or daily life. With the advent of photography to the masses, current events were documented in a different way and historical events of romantic past (Rome) was a way to express.
The 1800s again brought the advent of the Romanticism period. Beautiful massive scenes of crashing waves and vast skies over the subject. These are amongst my favorites and can be massive works. The 1800s also brought a huge array of other styles as artists attempted to come to terms with photography and it's availablity to the general public. This includes Academic art, Impressionism, realism, and several others.
The 1900s brought the big changes to art as the general public may see it. Early in the century was post-Impressionism and symbolism, both slightly traditional but the catalyst for what was to come as realism generally was discarded for imagination.
The early 1900s ushered in expressionism (Van Gogh), Cubism (Picasso) and futurism. A change in how art was seen generally and the turn to seeing art as a true glimpse into the artists mind and vision.
This quickly moved to Abstract, Dada, Surrealist and Deco. Pop Art was the normal progression at that point which includes Warhol and his friend Basquiat. Pop art is currently near the top on collectibility as those billionaires collecting fall into the age group of appreciation for those artists. I think it is a surprise to many that what they consider "art" has not been the common period subject for well over a hundred years.
at this point we must realize that amount of trained subject artists is high and thus not as collectable in the art world. The success of a painter like Thomas Kinkade shows that the public does still like art that they find comfort in its ease of interpretation and beauty but will never find a place in a true art museum. Does that mean it is bad? Absolutely not, just not a historical piece nor a piece that inspires thought or discussion.
Graigs works are beautiful and I hope to have one for my baseball room soon, but I am sure he would not take offense in me stating it will not likely have a place in traditional art museum as subject art. I do hope it does find space in somewhere amazing like the hall of fame where it would be greatly appreciated.
As for appreciation, I think there would not be a loss of value, especially at the amount of pieces he produces. If he were to license the images as prints or overproduce it may, so I hope he does not. As it sits, he could easily double or more in value as sports collectors will always love good work. So it is absolutely a good investment, but don't spend your millions yet.
...And if you made it this far in this post, thank you.