Rejecting them leads them to getting their grain elsewhere, and signals that we value Sparta-supporting Taras over Athens. When the grain trade evaporates and the generous friendship of Athens is rejected, so goes the goodwill. All of a sudden, Eretria becomes an unknown; whose side will they come down on, should it come to it? You suggest maybe piling onto Athens with Sparta to preserve balance of power, and that's precisely what they could not tolerate. This makes them far more likely to take hostile action down the road against us, if their citizens are not as fond of us, they are relying more so on the Hellespontine grain, see us cozying up to the Spartan side of the spectrum and aren't sure where we'll fall. Athens will take action to secure its needs; I'd rather be paid for it than have them decide to risk taking it, especially when their payment can be used to make us a tougher nut to crack. Also, should they shift their vital trade away (again, a third of our routes) that cuts down on what we can build and maintain in defense.
A deal with Korinthos seems foolish. They have their inclinations towards reclaiming our new league members and seizing Adriatic trade, and would have no reason not to turn on us even in the unlikely event that Athens is overcome because they vastly outpower us, we have bad history and they want what we have. Only Athen's efforts have shielded us from them; what do you think would protect us if Athens falls? If it's friendship from a common victory, I don't see why you would trust that of Korinthos any more than Athens considering our histories.
We're not on Athens tier. It's OK to suck up to a big power, so long as you stay mindful and find a way to benefit. We can use that stacks of silver from enhanced trade to make ourselves more powerful; we could more easily backstab them later should we feel it warranted to do so because our league will be more developed, populated and prosperous (time is our ally as a settler state), our ships are raising in number and I find it likely that an arrogant Athens would bite of more than it could chew in struggle against Persia or something. Plus they run risk of more revolts, as they grow larger there are more opportunities to get bogged down in various cities deciding they've had enough and revolting while our lighter touch is not as prone to suffer like that in our own league.
I think that rejecting Athens for the principle of it to "not be a puppet" will lessen our capabilities and dampen our long-term growth. The trade wealth (again, Athens is a third of our trade and half the luxury items) can easily shift away as threatened in the update and leave us able to buy less and afford to maintain less of a navy. Please let's not shoot ourselves in the foot to make a statement that we can't yet back up.
