My argument wasn't that Rhegion would care too much about the Adriatic stuff and more that Rhegion would care about us helping their rival...
The thing is, we are "helping their rival" in a way that doesn't actually hurt
them, because their rival still vanishes into a puff of smoke and is never seen again.
It's a mistake to imagine Rhegion as 'hating Kymai' in the sense that they actively want every citizen of Kymai to die. They're business rivals, not sworn enemies. They're not going to snarl at us with hatred for decades because thanks to us the Kymai refugees
aren't dead.
If in the process of "helping" Kymai we
evacuate Kymai and the city is overrun by Oscans who have no interest in competing with Rhegion, Rhegion wins. Sure, some Eretrian traders may gain a bit more navigational knowledge of the Tyrrhenian Sea and make some money in that body of water, but no realistic level of Eretrian trading activity in the Tyrrhenian Sea can ever be a threat to Rhegion's primacy in that sea. It can never hurt them anywhere near as much as the relocation/fall of Kymai helps them.
It's like, the Greek city-states generally don't love each other and will let each other fall without lifting a finger. This does
not equate to them holding some kind of lasting grudge against anyone else who dares to
help another Greek city-state.
And if we really want to move large groups of people out of the city I doubt we will be able to avoid using convoys, especially since it would be sensible to use our fleet to protect our ships during the rather long and perilous journey and the moment we even think about using our tiremes we are practically forced send a sizeable marine contingent with them to guard our ships when we beach them as is common practice.
I mean, convoys, probably- but
big fleets, not so much. If nothing else that's hard to organize and means that the number of people who might be killed by a single storm goes way up.
Honestly I think this is kind of a terrible argument. If we want to demonstrate our strength and confidence there are a bunch of more practical things we could do closer to home (like beating up the Dauni) that also have more tangible benefits to our wealth and security while not compromising us against dangers from those same vectors.
It would be a demonstration of naval strength that is (and this part is important)
visible to the Italiote Greeks.
If we conquer the Dauni, they won't see it happen. If we solidify our rule among the Messapii, they won't really see that happen either. Even our more maritime options like "colonize proto-Venice" or "extend our colonies on the east coast of the Adriatic" are out of sight and out of mind for them. Remember that it's not for nothing that they don't even really think of us as being part of the same geographic region as them.
If we want to largely avoid engagement with the rest of Italy, while building up a power base centered on the Iapyges and the Adriatic trade, then yes, not rescuing Kymai will further that isolation.
If we want to engage with the rest of Italy, then rescuing Kymai is a good way to demonstrate our strength without making enemies- a display of what our stat system calls Magnificence.
Personally, I don't think we really have the option of
not engaging with the rest of Italy. Not in the long run.