Why I don't like the idea of paying for extra retainers.

[Felis]

But, you don't need to sells extra retainers for that, actually, a player can register 2 standard accounts and get a total of 16 retainers.

This

standard + standard (30$ ), standard + Legacy (25$) or Legacy + Legacy (20$).
If you only count the 16 retainers on the 2nd account as extra retainers then it would be 1.88$ (S+S) - 1.25$ (L+L) for each extra retainer