The point isn't how many zetas you have... whether it's 1 or 100. They aren't difficult to get if you devote the resources to them... but it's time consuming. If you spend your ship currency only on zetas you get one about every two weeks on average.
Sure I have a good amount of characters zeta'd but I started early on it and have spent all my ship currency exclusively on zetas... I like to try new things. That's what makes the game fun for me.
They were great initially to help address the power creep by giving older characters that became somewhat obsolete new abilities that made them viable again. Now every new character "needs" a zeta... or two.. or now three in Luke's case.
It's literally impossible to stay even remotely close to keeping up with the curve that is being created. Sure you can save them up... but for what and until when? What if you went "all in" on the Ewoks and then they release Luke... well that sucks... or you go "all in" on Luke and soon enough some new character is released that is bigger and better and nullifies all your efforts.
I think that ideally any fully zeta'd team (empire, sith, Jedi, resistance, FO, Ewoks, etc...) should be able to compete with any other in a rock, paper, scissors kind of way... but currently it doesn't work like that. They just release something new that is stronger than anything else and shifts the balance dramatically. Arena currently is probably more varied then it has been in a while but it still boils down to a very limited number of viable teams/ characters is you want to be competitive.
I don't really regret any of the zetas I've put on characters. Some are clearly far superior to others but regardless it's fun to try them out and experiment with them.
That being said... being able to remove them for a significant fee or having them accessible in other ways wouldn't be such a bad thing.