Perhaps you are right.
On the other hand, say you are halfway or so, and an angry email comes in, threatening with a lawsuit unless you remove the graphics, since they contain copyrighted pixels.
What would you do then? Throw all your work away? Now that would be a waste of effort.
In my view, originality should be good enough to defend the graphics in court should the need arise. Doing work under the assumption that nobody will notice or bother enough to send that email does not seem a not very good way to start.
I do hope it will never be necessary to defend the created graphics in that way, but who knows what happens in the future.
If you re-use those graphics, how much time will you gain, and is it worth it?
I suck at painting pixels, but in my coding I prefer quality over quantity. If you give me the choice between making some changes in existing code, or writing new code that I can make in the way I like it, I'll take the latter any day, even if it takes much longer. In that way, I can reach the best possible quality.
Whatever choice you make, the project will take years. The point however in my humble opinion is to have fun while doing it. If it takes more time, who cares? You'll have fun for a longer time.
So rather than time, I believe you should optimize for surviving angry emails and for fun for the people doing the project.
I cannot do pixel art, but by making a language that makes entering animations easily, I hope to help a bit.
Edit: One way to make the project more feasible is by making it possible to have the new graphics gradually replace the old graphics. In that way, the progress is much easier to see by everybody, and you may get more people drawing.