It's actually already behind a paywall for most people and don't need to be achieved with a bundle.
It's behind a paywall because you would have to get good results in Ships to obtain zeta or having played so long that you achieved the ship store.
It's also obtainable from GA, which eventually is more accessible for people who payed in this game or directly had a very good start, focused on essential characters, etc...
And it's a reward from TW, an event where powerful guilds with numerous squads perform better, where obviously, the more GP you have the better the results are but still not going along with the GA, so you'll have to be in a very good place for both if you want to get a lot of zetas.
And it's achievable through events or first time missions in fleet battles. Anyhow, it's a long grind, more accessible for people who paid at some point if not continuously.
So no, please don't put a bundle for this kind of material and keep a kind of balance around this game that is going nuts.