4000 Points in spacetime shop seems an out of mind price considering how few points we get now. If in the future they add other ways to get them, like they did with ranked battle points, ok, but as they are now that price is rediculous.
Anyways i like the idea you were talking about. The problem is that what petrichor said is true. You cannot go for ninjas in group shop until you need medium refinements, you cannot go for ninjas in spacetime because if not you can't buy purification runes, advanced threads or deluxe tactics boxes. The only shops where you can think to buy ninjas in your late game are survival, arena and ranked ones.