I think it might just be your playstyle
It is a fair playing field but purely spending to get ahead is an act of self harm. Refines and other things are based around an rng model so it is not best to just spend but spend at points that reduce this risk of failure such as when refines are 50% of the shop value or below and not around shop value. Without realization of risk it is pretty short sighted to say paying more is better.
I feel that you assume the value of a ninja is related to how much one must spend for the ninja and this is an error in judgement. Ninjas are not that simple, such ninjas as kimimaro for example would be sss or godtier if we had a tier system since kimimaro is so good at doing his job, and you know what he is probably 5 star for every user currently playing.
In addition, most people wont try to collect all the ninjas and play a selective style. Therefore, they will get a ninja from recruit they find great and build a team around it and * with it for awhile. The game isnt forcing u to buy the newest ninja at every release. Paying a premium for a "collectible" that you could of waited for is kinda your fault not the games.